SCSI is dead, buy SAS for super high performance. Expect 4x the price of SATA.

SAS is preferable for databases since writes are performed to the disk in a safer manner than SATA (cache on the drive is smarter and reports written only if it can be safely written with a power loss).

That said, SATA with a 3ware raid controller with an installed battery backup is probably just as reliable in the real world.

In enterprise environments, SAS with battery backups on the controllers is what gets used today. But thats major $$$.

For a 'forum' you just want high performance, losing one post every 100 years won't make a difference. So just use SATA raid 5 with a 3ware controller. Read performance is excellent on Raid 5. Writing is slower but a forum does mostly reads.