You should use the M series if you're having a heavily used database. If it is lightly used, the T3 is supposed to have a much better thread performance than the T6320s.
Remember, the T-series blades which are CMT give you a great thread performance, but suck on process performance per se. So if Sybase is more "process based" than thread based, it'd be smarter to go with M-Series if it's heavily used. If not, go with the T3's. They're cheaper than the 6340's as well :)
Also -- it depends on what T3 blade /server you go with. IF you're going with a large machine (the T3-4 processor) then you'll be fine.
If you can wait, you should wait for the T4 series servers to come out, which based on all the chatter, should be out soon and is supposed to have significantly better thread performance and performance optimized for Db!
I'm currently running a 11g R2 RAC cluster on the T3-1Bs and its not bad. Our db is not heavily used, but we're using LDOMs and SAN connected storage. So far its not been a problem for the RAC cluster.
However, for heavily used Dbs, a T4 would be much better, and in the future the T5 chip is supposed to be switchable with a T4 motherboard chipset. So if you're buying new, it's something to consider.