Thanks for the replies guys. As someone else mentioned, yes my IS300 is RWD and I currently have Dunlop M3s on them. I have not driven in deep snow yet with my IS because I've only had it since last March, but in light snow the IS feels very safe. I know that the 330i will no doubt outperform the IS in the summer months...that's primarily why I bought it (and it also has an aura that the IS lacks); however, if the 330i will be as good as the IS in snow, that's all I need for reassurance.
BTW, any specific e46 years tend to be better than others or no? For example, with the IS300 the 2003-2004 models were the best of all other years from 01-05 because it had the most features and still retained LSD, whereas the 05 models didn't.
Any help is greatly appreciated.