If you have a fairly simple midi controller setup and don’t require much customization (and are willing to work with Ableton’s workflow), Ableton will probably work fine (and might even be preferrable).
Ableton has tons of native controller support. So with Ableton alot of controllers work right outside the box.
Ableton also comes with a bunch of nice synths and effects (especially if using the Suite version).
Ableton falls short in terms of customization of setups. Ableton tends to be more cumbersome when setting up “dumb” midi controllers. Gig Performer is alot more customizable and “Dumb” midi controllers in Gig Performer are powerful devices. Ableton has pretty decent midi controller customization but it is very project based and things can’t dynamically change like Gig Performer (where each Rackspace, the controller can utilize a very particular role). Ableton has some script support via Max4Live to make controllers more customizable, but in Gig Performer these basic controller mappings are more made available to the common user. Gig Performer has scripting support as well but you can often do alot in Gig Peformer without needing a script.
Probably the biggest differences is that Ableton follows a mixing console (Global project) concept. This is a fairly easy to learn concept but can be limiting in certain live setups.
In Gig Performer you have a Rackspace concept which is totally customizable by the user. Each Rackspace is kind of like an individual Ableton Project which can be totally different from song to song or even song part to song part. This is the automation piece where Ableton falls short.
That said I think if you are primarily playing stems and are only playing minimal keys, you might like Ableton better (or at least it should be able to cover your needs).
I enjoyed working live with Ableton (when only using one midi keyboard controller).
The limitation I ran into was when I decided to add midi controller #2 for my top keyboard. Ableton (still to this day) has no good way to manage multiple midi controllers for live use where you can move around to different tracks using the auto arm feature. If you do this with Ableton, the auto arming will conflict with you other board. You can probably do something similar using Instrument Racks but you will lose alot of the customization this way of Ableton, and this is an area where Gig Performer was designed specifically for where Ableton was not. Ableton’s auto arming actually works great for Studio Production and Song Production but just wasn’t designed for what I needed it for and has no elegant way of acheiving what I need, where Gig Performer is made for multiple midi keyboards in mind where you can have two, three or however many you need.