Hi, Wendy,
First, the platform. At present, both Windows and Mac boxes will work with ModelTalker voices, can be used for voice banking and can be accessed via eye gaze interfaces (Tobii or Eyegaze Edge for Windows, Eyegaze Edge for Mac). Both can also be used for dictation apps if your hands go before your voice.
I'm biased toward Mac (all I have ever used at home, but have often had to use Win at work) but I'd use what you like. If you're agnostic, I'd go with a Mac simply because of the ease of maintenance, but that's me.
Second, the form factor. It's hard to look ahead in ALS even a couple of years, since everyone is different as you have seen. I'm typing this on the latest (12") MacBook, using the same stand horizontally that used to hold my husband's iPad mini vertically, which he used for watching video in bed/the sling. The reason I can change the angle by that much is that (1) I'm using a floor stand and (2) the new MacBook is very light (2.03 lbs). The same logic would apply to a clamp system for a table, bed or wheelchair.
Then we get to whether at that point for computer control, you're using a regular mouse or switch because you have some muscle(s) that still works, a head mouse (SmartNav), a tongue mouse (Tetramouse) or an eye gaze system. All this plays into what height your screen needs to be, what's attached to the laptop and things like that.
But no question you have more flex with a thinner unit (easier to raise/angle/transport), so why not get the thinnest? That would be the MacBook I'm typing on. Better ones (2nd gen after they "rebooted" the form factor last year) in terms of speed are due out in the spring, and so the prices for this one are already falling; I bought gently-used anyway. If you are not in a rush, you could see what April brings and whether you like the new one better.
If screen size will be important to you for some reason, e.g. you do a lot of graphics work, of course, the MacBook Pros go up to 15" but with a Retina display, I don't see any noticeable difference between this one and the 15" MacBook Pro I handed down. Since your ability to compensate for glare diminishes with being unable to turn your neck, I would definitely recommend either a Retina or matte display, not the glossy coating that is standard on many laptops these days.
If you want to share what apps you envision using, what you like doing on a laptop, I might be more helpful... and if you have a mobile device or laptop now, I'd be happy to show you what it looks like on a floor stand via video, and how's the Trilogy or other BiPAP coming?
Best,
Laurie