personally, id go 16v all the way.
i currently own 2 series 3 16v's, but don't take this as my bias, i have had many other engines and alfa's, but don't actually find the 16v lacking in low end torque compared to the 8v at all,
but where the 8v starts to tail off from mid to top, the 16v is really coming alive, and makes hard driving much more rewarding when you want to.
never had any engine reliability issues whatsoever, the one i'm using daily has around 120k miles on her, sweet as a nut.
just the gearbox synchro's as mentioned earlier in the thread, particulalrly 2nd gear, is just terribly weak, and cant start to crunch at ANY mileage from what i gather, my own low mileage cloverleaf hasn't covered 40k yet but has been crunching 2nd for ages!
gearbox synchros and rust if from a damp country like blighty, other than that, great cars, great fun, you'll love it, 8v or 16v according to your flavour!
cheers, good luck
shrew