The thread on sequencers was getting heavy on other items (sampler, etc) - so it may be worthwhile to have a dedicated thread for comparing the two keyboards.
Just a few items to kick things off.
Fantom has a more fully capable on-board set of sequencers. Montage has a recorder with limitations and no editing.
Fantom has balanced audio inputs (my favorite XLR+1/4" TRS combo). Phantom power is supported and each channel (L and R) gets its own level trim. Montage has unbalanced audio inputs. No phantom supply. Level gain knob is applied equally to both L and R. Montage's knob is on the front face - Fantom is behind the keyboard. So probably not something you will use much on Fantom past set-it-and-forget-it.
Fantom has XLR audio outputs (my favorite cable for live use). Montage has balanced - but TRS
Fantom allows controlling all 16 zones with local keyboard. Montage allows only 8 zones under local keyboard control.
EDIT: worth noting that Fantom's zone, made up of a "Tone" can have 4 different "partials" - so some look at this as being the same as Montage since Montage's AWM2 supports 8 elements and if you see "partials" as elements - then 16x4 = 8x8. However, the difference is that all sounds on Fantom work with this - so a single zone is a fully-formed instrument which all are done within 4 partials while Montage gets 8 elements. I think, given this, it's a false equivalency - although worth mentioning. Both Fantom's PCM (sample) based engine and the virtual analog support 4 partials.
Fantom supports "seamless" type Performance switching with less limitations than Montage. Montage, and only when supported, you can only switch to one next Performance while holding the original sound. With Fantom, you can switch many times (seemingly unlimited - more than 5 as demonstrated) while the original sound is still sustained. Also, all 16 zones are supported. Fantom has no limitation based on zone usage - as Montage only allows the 1st 8 PARTs to be used by both the original and final Performances in a switch.
Fantom has a touchscreen operated joystick-like fader between 4 different sounds with ability to record a travel pattern. Montage has superknob which is different. You can realize the same sort of thing with appropriate programming - but also have superknob target many other parameters.
Fantom appears to have ability to chain performance equivalents. Closest Montage has is sequential position of Performances in live set and using footswitch to advance to next Live Set Performance. Maybe a wash here. Would have to learn more about Fantom to know advantage/disadvantage.
Fantom's GUI allows more "touch and drag" operations - which enables editing curves by "drawing" on the screen (such as ADSR envelopes, cutoff curves, etc). Montage doesn't have any touch/drag support so editing is done more indirectly (increase/decrease individual parameters).
Fantom's pitch wheel and possibly other controllers support 1024 values. Montage's pitch wheel supports 127 values. Use of Fantom's controllers results in less "stepping" - more smooth results than Montage. There's a thread on Montage's issues with pitch bend.
Fantom has some dedicated knobs for items which are typically "deep edit" items. Cutoff, Resonance, A/D/S/R, patch select. Montage you can assign different assignable knobs for some of this - but Fantom's more dedicated knobs integrate a bit differently with how different sounds can be selected to apply to their fixed-function knobs. Slightly more "dynamic" than Montage's approach for these functions.
Fantom has an engine supporting modeled pianos. Montage doesn't have this.
Fantom has a virtual analog engine. Montage doesn't have this - but FM-X is the trade - Montage has this and Fantom doesn't.
Fantom seems to have more knobs/buttons. There are the knobs similar to the assignable knobs in Montage under the faders. Then there are a set of knobs in Fantom below the touchscreen. These sport my favorite knob feature to have a push-button integrated so they take less space and can do more. There are the other knobs mentioned before for ADSR/etc. Then there are the pads which can take on many different modes (DAW control, sample playback, etc).
Fantom has tight integration with Mainstage where the Mainstage screen will "show up" on Fantom. You can see the VSTi controls on the Fantom screen. Montage's integration is less. More geared towards having Montage (knobs and sliders) controllers assigned to controlling different things in your DAW - but no other integration.
Fantom ... there's also vapor-ware "rumors". Where reviewers mention that more synth engines are to come in future updates. Not sure what will come of that. Montage does have this - a rumor of a new engine in the future. Relatively speaking, Fantom already starts with more engines.
Fantom has CV/Gate outs. Montage doesn't have this.
Fantom has more aux audio outs. Helpful for click track and other uses. One extra output compared to Montage.
Fantom supports 3 USB ports for external MIDI USB-MIDI controllers. Montage doesn't have this. Technically, the USB port for Montage is for flash drives only.
Fantom has 2 MIDI outs (or 1 out and 1 thru). More possibilities perhaps integrating with external gear. Montage has 1 MIDI out.
Fantom has a joystick control (pitch and mod in one controller) or wheels. Montage has wheels.
Fantom has a sampler built-in. You can edit loop points and such. Montage doesn't have any sampler features. Currently, Fantom doesn't support sampling/editing multisamples on board. The sampler is "dumbed down" to only support loading in one-shot samples for the multi-pads. The keyboard has a lot of beefy support for audio input (balanced inputs, phantom power, XLR or TRS) - "expensive" stuff to only support pad-triggered samples. The vaporware promise/rumor is that multisample editing is to come later. I would think, given what's there, this seems like a reasonable thing to anticipate. That said, Roland's track record of firmware updates and support-after-the-sale has not been stellar. As many complaints I've seen against Montage on this front, Yamaha did a great job of delivering. So great that I think it started to setup an expectation of ultra-frequent updates. I would say relatively, Yamaha (Montage) wins in this category although there's no real data on Fantom yet.
Yes, none of these matter if the keyboard sounds like a tin can stuck the bottom of your shoe. Fantom's V-piano engine sounds great to me. The synth sounds are Roland - they have a characteristic sound I do not personally gravitate towards (which is why I've gone the Yamaha path). But they sound fine and balanced with the other sounds. I think the sound is no slouch and there is vapor-ware promises of updates to bring more/different sounds to the table where the current release sounds are more derivative of past products.
There are more differences to possibly highlight. Those are the main ones I can think of to start the ball rolling.