I've been to both A.C. (a handful of times) and Placencia (once, this past January). I can't speak for the roads at the time you're going, but when I went they were fine.
We spent the first half of the trip up in Corozal, and drove from there all the way down to Placencia in one day. The group that went (two couples) all agreed that the drive was one of the best parts of the trip--portions of the Western, Hummingbird and Southern Highways which you'll likely take are beautiful.
Placencia doesn't have as much going on as Ambergris Caye, but some people would say that's a good thing. Also, the beaches are higher ground and generally nicer. However, the sand is much coarser than the fine sand out on Ambergris caye.
Foodwise, we thought Ambergris Caye was better, both in terms of quality and in choices.
We did like the fact that Placencia was much quieter than A.C., as well as the fact that it was within driving distance of some neat mainland sites.