I love my 280GB, but there is no way in hell I would drop what Intel will want on either of those two larger drives. Diminishing returns in my opinion. The 900P is awesome as an OS drive as it makes the system super responsive and that is all I really want out of it.
SC codes heavily subsidize the smaller 900P drives as a boot/program volume rather nicely, but that will only last so long until they stop coming with codes or all the SC whales are fed. I predict the former occurs before the latter as those whales are still hungry even after years at the pre-launch trough.
The 2.5" work fine with either the included m2 adapter or adapting the u2 cable to m2. You're in MacGyver territory on the second method but still electrically valid