Based on my experience,
I have owned over the last 6 years or so... an e30 325is, and E36 325is, and now an E46 325xi.
I commute 100 miles each way weekly for my job, and I live in southern ID where we get a lot of snow..
With the RWD models..without good snowtires...and I mean GOOD snow tires like michelin artic alpins or blizzaks, the rwds are a chore to drive in the deep snow..bordering on dangerous...right after I got the e36 I had to drive it after a snowstorm with just its yokohama all seasons on it...wow...bad news.. I had to go really really slow...after I got the artic alpins on it it was doable..not excellent but ok. Also in that first little bit before we got snowtires my wife did $2000 damage to it by trying to turn into our driveway a little too fast and plowing into the mailbox. If you get any kind of snow at all, with the RWD models I would say that snowtires are a MUST.
I just got the xi, and I havent had to drive in a really bad storm yet..but, in just playing around with it its a LOT better, more stable, and safer in the slick stuff, especially with the DSC engaged. However...I went and did some canyon carving the other day, and I really kind of miss the punch of my old E36 5 speed...so i guess its all a trade off, you have to decide whats more important to you, and how bad the conditions are that you are forced to drive in.....mine get pretty bad sometimes...hence the xi this time around.