The spindrift is also great. My only complaint on that particular piece of gear is that it takes some time to take on/off (you really want to take about 15 minutes and do it inside at home) so it works best to put it on for the whole winter season. Not a big thing, but it takes away just a bit of the flexibility of using it.
It'll bring you right down to around 20*, so if you camp just a few times a year below that, it's probably worth going with the 20 and supplementing. That way, you save weight/space for all the other trips about 20. If you winter hang often, though, the 0* is worth it.
Personally, I went with the 20* as I only camp a few times in winter - and I have the Spindrift, which adds 10-15* of warmth to the setup. I like having the flexibility of adding pieces to the system instead of having an overly heavy piece I need to carry in the summer. The lighter weight of the 20* lynx is another reason I really like it - it's still full length but I save nearly 10 oz compared to my Incubator. That just about offsets the weight of the poles, making it nearly comparable to the carry weight of my Blackbird/gathered end setup.