I'm 5'11"/175/sz10 and I love my Diesel 65 that I got almost exactly a year ago as my first whitewater boat. Very stable, maneuverable yet fast, comfy, room for gear (or even for a little camping), surfs well and forgivingly, and is good for when you want to step up a notch, and then another, and hit bigger drops and/or bigger flows. I could have gone with something more playful and with less volume, but I feel that would have been better for playboating, and not as good for serious river running fun like I've mainly been doing. You can always add an edgy little playboat for cheap later on (the playboats are easy to get used) when you have a little more experience ... I just got an old Kingpin 6.3 today

-- now I have my big water/ big rapid boat (that was stable and good to learn in) and my low flow/ play spot/ fun toy.
The Mamba 8.0 might be an even better fit for people our size, I really liked the smallish 7.5 I tried in the pool once. GT 8.1 might be too big and floaty, maybe a used 7.8. If I could buy a new boat right now, as a great all-arounder I'd probably get the Pyranha Stretch M/L. I'd get used for a first boat, though -- others to consider: Liquid Logic Li'l Joe, Riot Booster 55/60, Riot Nitro (edgier/trickier but surfs great).