[OverDrive]

I test drove both when I was going to plce my order, and they both seemed pretty equal performance wise, IMO, but if you are wondering how the 0-62 times are affected, the xi gets a better launch and therefore is faster to 62 than the i. However, after that, the i will be faster due to it being lighter. The i is closer to 50/50 weight distribution than the xi for abvious reasons, therefore I'd say its more nimble around the track.
Now the AWD system. Well its not as good as the EVO's since the xi version on the 335xi doesn't have torque vectoring which means that it can't send power to individual wheels, it can only send power to either of the axles. However, it is very intelligent and since its made for BMW's, i think it allows for a bit of performance together with safety aspects.
I think to get all of that power on the ground, its better to get the xi, because then you can get coil overs, suspension and turbo tuners to make it more nimble and even better than the i.
Good luck with your purchase