I have written countless posts on this in various threads, as someone who has supercharged the 6G75. there are eccentracies with the set-up on the 4g that would probably not make it as hard to do on another car.
that being said it comes down to this, as far as for the 4g:
the Turbo (assuming all is done right, tune, supporting parts, etc.) will make more power in general, and the BOV is of course very cool. HOWEVER, for our cars specifically there is not a good pre made package - so you are stuck doing it all yourself - not an easy task for a novice weekend mechanic. they are probably slightly more complex, require intercooling of some kind, and all of this makes getting a perfectly tuned and working safely turbo set up a tall order on the 4g, especially for the GT. The two GS' that have turbos that I know of, (elemenopee & b-shue) both have problems that have them sidelined at the moment. HorseEater has a sweet twin turbo set up on the GT, BUT also has problems w/ the tune (horse correct me if I'm wrong) and needing other parts to be upgraded to get it REALLY producing the power it could/should. One of these issues on our car is valve float.
as for the supercharger - at least there is a specific package out there for us, and yes I know RIPP doesn't make it anymore - BUT you could probably find a used mounting bracket and drive shaft (the two main things you'll need to get the blower working) or even maybe get RIPP to sell you them. the blower is just Vortech, whatever trim you want. so someone wanting a blower could get one, put it on there and make it work, same as trying the turbo. STill need supporting parts and a good tune.
BOTH have struggled with the fiesty ECU of this car - the AEM F/IC will let you overcome some of the issues, and at least deliver enough fuel. If you are really smart you could figure out how to use the o2 manipulation offered by the device, to get some of the o2 codes to go away. To me, though, the sound of the SC is off the hook. It's not as loud and a little more subtle, but it sounds like a damn UFO coming down the street. there are 2 different cool whines the unit makes depending on what RPM you are at, and it's sweet as hell when you hear the RIPP by-pass valve open/close. (sounds like a sword being unsheathed). I love mine to death, and feel it defines my car. Even not going into boost, just pushing that extra air in there is awesome and adds power every single time you press the pedal down. A turbo will lag a bit before it starts making good power for you. the other thing to do is get the reflash from Hackish to make this work. I personally have spend THOUSANDS on electronics/tuning trying to make mine run perfect and have not yet had success - BUT I am about ready to do the reflash which SHOULD clear all this up once and for all.
VALVE FLOAT will kill both set ups going above 7 lbs or so. The only way to fight this is to spend $$$ at deltacams or somewhere similar to get your valves upgraded to oversized and hopefully a 3-angle valve job and cam regrinds/welds. Very expensive, but no doubt worth it if you are serious about getting up into the 10 - 15 lbs of boost on our high-compression engines.
OTHER PARTS & UPGRADES are FOR SURE needed for both - like suspension, tires, clutch, LSD, headers / turbo manifold, high quality exhaust system, engine bushings, suspension bushings, strut bars and sway upgrades, intercooling (via meth injection for SC and a FMI for a turbo).
well, I was trying to make this a simple post, but wrote war and peace instead. there's probably a lot more to be said, but I'll leave it there for now.