Hey Basista, welcome to the forum
First of all, the Sub Phatty doesn't have any arpegiator. So if this is a
sine qua non condition for you, just be aware that this guy should be added an external one (candidates exist, I'll let my fellows advise about that).
Now.
The number of keys may matter, indeed. I play simple basslines in a rock-electro duo and I own a Little Phatty. For weekly rehearsal, I use my mate's Sub Phatty. My LP's 3 octaves are enough, 2 octaves are sometimes short, but generally speaking, it's ok. Plus the transpose buttons are on the panel and you can develop cool manual arpeggios using them
About the functionalities, the Sub 37 goes way further than the Sub Phatty or Little/Slim Phatty in terms of sound design. If you're not afraid of being absorbed by making sounds, go Sub37. I don't have it but read only good things about it (other forum members will certainly chime in). If you do prefer focusing on playing and if complicated sounds are not a priority, you'll get good, expressive, easily programmed sounds out of any phatty.
The interface of the Little/Slim Phatty does not have as many knobs as the Subs, but they remain very usable, even live. You may want to try them all to determine what kind of panel you prefer.
About presets, the Sub Phatty only has 16 of them. Sounds ok to me, but too few to many. You could take this as a guitar/bass: pick-up selector, volume knob, tone knob, one pedal... that makes 16 presets, more or less
Soundwise, the Sub Phatty and Sub 37 share the same sound engine (oscillators, filter, Multidrive in particular). I do prefer the LP's sound that is less... straight and rigid to my ears. The tuning of its oscillators can get a bit more approximative, and that Overload functionality's got more meat than the Multidrive.
As you'll be playing it live, weight: about 22lbs/10kg for the 3 octave versions, maybe 17lbs/8kg for the Sub. Modules will be lighter but will need a control keyboard: less sexy to me (that'd open the doors to the Minitaur, though).
Best peace of advice: try to get your hands on them, listen to the feeling you get out of it
