Here are some general differences between the two:
If you have large hands, I recommend going with the Flash 2. It's large enough to fit comfortably in the palm, where as the Twitch 2 is much smaller and harder for me to comfortably open and handle. (Because of this, if I'm looking for a Twitch, I actually prefer the Twitch XL.)
The Flash 2 has a lighter feel- This is mostly because of the Zytel handle versus the Aluminum Handle of the Twitch 2. (There is also an Aluminum Handle Flash 2 option- it has a significantly higher price point, but it's heavier & has a *slightly* smoother opening action, in my opinion.)
The handle on the Flash 2 has a checkered texture and is much thicker than that of the Twitch 2, which is very slim and smooth... If you're looking for something with better grip, go with the Flash 2.
Both knives include the spring-assisted opening mechanism, although the Twitch 2 has the added feature of a pass-through lockbar with a kick. This "kick" gives an additional option for opening, if you find thumbstud opening cumbersome.
As far as the two different styles of release, it's really all about whether you prefer the release on the side for a Flash 2 or a lockback on the Twitch 2. I've never had a problem with either.
If you're interested in a half serrated option, or want a black TiNi coating, go with the Flash 2- the Twitch 2 only comes in a Satin finished straight edge.
Both knives include a safety to lock the knife closed, and both knives have AUS8 steel blades, so unless you have serrations either shouldn't be difficult to sharpen with the right tools.
Hope that helps!