Don't forget that all current SAS drives are the same as the U320 drives with only a different controller. So performance will be about the same, since no drive comes close to the 320MB/s that U320 offers.
When we look at big RAID setups with a lot of drives, SAS will be faster because the drives don't share the same bus as with SCSI, for these big setups you really had to have Fibre and now SAS is also an option.
Also SCSI is really mature since it has been on the market for a long time, SAS is just appearing on the market and it will take some time before things will be really optimized, first they have to get everything working then later they can focus on optimizing.
For me buying a new SAS setup with a few drives is not because I think SAS will me much faster then SCSI, because at the moment speeds are comparable. It is about investing in a technology with a future since SCSI will be phased out in favor of SAS.
I don't know about Adaptec's stability, but in all the years I now work with SCSI setups, I really learnt to prefer LSI over Adaptec stuff, I'm not saying the LSI stuff is perfect but since prices are comparable I think you get better stuff with LSI, so I will be buying a LSI SAS controller and I'm not worried that it won't be great, because I know I will not be disappointed.