Nebraska 330i owner here: Remember that the Xi won't stop any faster than the Ci. Getting stuck is inconvenient; ramming things is worse.
Snow-tires, in addition to making any car stop better, also make rear-wheel drive vehicles drivable in winter. Not as good as FWD or AWD, but drivable.
So if you really want the Ci, live a little and buy it! Get snow-tires for the winter months, and accept that you will have a little less "go" power but plenty of "stop" power, and then savor the drop-top once warmer weather comes.
If your major concern is getting stuck (and it might be up there), then go for the Xi. All-wheel drive with snow tires can be a blast in the winter, because you can smoke SUV's away from the stop lights. SUV owners hate this, because they like to think they own the winter streets, when in reality, AWD with soft sticky snow-tires is unbeatable!
So my point is get snow-tires either way. Then, with the Xi you'll be Winter King, or with the Ci you'll be "just okay" (but drivable) and ready to drop that top at the first sign of Spring.
I recommend tirerack.com, and get them pre-mounted and balanced on steel wheels with some cheap hubcaps for winter. Then you can just pop them on yourself when that first big snowstorm hits.
Once I found out what a difference they make, I bought them for all 3 vehicles I own (BMW 330i, Maxima, and 4WD Ford Ranger).
The FWD Maxima with snowtires is unstoppable in winter. The 4WD truck with snowtires is 2nd best (yes, you got that right) in every respect. I'd take the Maxima before the truck any day for security in knowing I won't get stuck. Last is the BMW with snowtires, although the snows make the difference between "drivable" and "staying in the garage."
Hope that helps.