i would say ssd is an excellent chiice for venti, the argument is less clear for fossil which us much more like a traditional filesystem.
fossil and venti do not have the performance if a modern filesystem bur an ssd can make them fast enough for most use (i dont stream movies from my plan9 system).
personally i have three disks mirrored, two rotating disks and, last year i added an sdd, they are irdered in the mirror so the ssd is read first but the winchesters are written first.
-Steve
As I know, fossil/venti file system is log-structured, so it may be good for flash devices, especially in extending life of flash devices.