just my humble opinion but i'd go with stalker. but i like torque more than high strung 4cyl. then again i'd love a 9k rpm 3 rotor turbo rotary. but thats not this discussion.
i agree that you should try and drive both and see your personal preference. i'm not a solid rear axle fan but that can be modified to IRS if thats the only deal breaker. neither one is likely to be perfect but if you enjoy one more than the other than run with it. and you should know yourself well enough to know what you like in a drivetrain.
torque vs. high rpms, roomy interior vs. small and cramped, etc. etc. i ride a V-twin sportbike(suzuki SV) because i didnt want the constant shifting of an inline 4, and i liked the 'heavy' steering as light steering on most sportbikes equate to twitchy to me.
also i know this will probably be debated but if you might modify whichever car the, the stalker i think wins for parts availabilty and
price. i know there is a lot of parts for a s2k but there are even more for gm parts and most will be substanstially cheaper. engine rebuild cost less even though it has 2 extra cyls, rear end has more gear and lsd options than you can ever want, most parts store will have parts it wont be a special order for eveything. then again i live 45 mins from Summit Racing though so maybe its just my point of view. and on average a forced induction motor will have easier power gains than na. i.e. change pully for more boost.
i'm obviously stated a little biased towards the stalker.
i've never owned either though and i'm building a locost with a mitsu 4cyl turbo so what do i know