V8, with the right tune, will get better gas mileage than the V6, and being an LS variant there are many more options for tunes, upgrades, etc. as previously mentioned. Also, the V6 is very limited to brand name aftermarket parts as far as it goes for engine upgrades with not much bang for your buck. Depending on your budget you could pursue Turbos or Supercharger but not much is available out there and at that point you might as well have gotten a V8.
Being a V6 owner I am still in love with my G8, but it is definitely more show than go. It still has the potential to hit 120-130 no problem, and get a lil sideways (on wet/snowy roads with stock 245/45r18 tires). But I'm not entirely sure if the V6 will be able to really replace your muscle car if it was a V8.
2009.5 LR Pontiac G8 GT Loaded
Solo Axle Back, Pup-Cats replaced with H-pipe, Spectre CAI, H&R Lowering Springs
Appearance and More:
Apple TV, VIM, Reverse Cam, ZL1 Replicas 20x9.5 40ET, 2 Alpine Type R 12's, OPT-7 LED Highs/Lows, LED Fogs, OPT7 Aura LED Interior Lights