In reply to midweekjolly2:
It entirely depends on the sort of climbing you're going to be doing. If its grade 1 and 2 snow, then I'd say a 60m 9mm single is ideal as you simply don't need the length of abseil afforded by a pair of ropes. Then once you get onto more technical terrain when a retreat is going to be more difficult, doubles come into their own. However, don't fall into the trap of thinking that doubles have a lower impact force, or that thinner ropes have lower impact forces. This simply not the case and more often than not you can get fatter ropes with equal or lower impact forces. The only real reason in winter to go for doubles is abseil retreat and drag reduction - most of the time you won't be placing gear in parallel (i.e. Two pieces next to each other and clipping a rope to each) which is when impact really is reduced as gear takes longer to place and is more difficult to find - you're more likely to be placing a single piece, so single or double is not going to make much difference. Ignore the ratings on the rope packet - they don't give the full picture as doubles are rated using a 50kg mass whilst twins and singles use an 80kg mass - once you correlate a double to this sort of mass the impact force on a single strand of double rope is not much different.
The other thing to bear in mind is that singles are MUCH easier to organise - when you are tired and its getting dark, not having a complete clusterf**k of rope is really helpful. So really the answer is you need both
But I have happily used a Mammut 8.9 Serenity and an 8.5 Genesis together (the latter being considerably cheaper) and it works well. Both these ropes have good double impact ratings (although not as low as Beals) and great longevity. You don't really notice the difference in diameter. The only downside is a little extra weight, but then think of the extra friction afforded by a fatter rope when they are wet or frozen.
For cascade ice? I've used twins, doubles, a thin single and a double and a long single. For ice cragging I'd go with a long low impact single - basically a sport climbing rope. For anything else probably doubles...