Having the databases on different spindles will most likely perform much faster if they are in use at the same time. If they experience busy periods that are offset from each other though a single RAID array will perform faster as a single database's reads will be spread over multiple drives (which it never will if it is on a single drive, of course).
RAID5 is often a performance bottle-neck for databases due to its pour write performance. If your array is a straight 5-drive array then every block written to disk is likely to result in three reads followed by the parity calc followed by two writes (the updated block and the updated parity block) and every pages that is updated in your DB will result in at least two block writes: one to the transaction log and one to the data file. Things will be a little better if you are running 4-drives-plus-hot-spare, but not much. This is why RAID1+0 is usually used instead of RAID5 for database work (or sometimes RAID1E or similar which, where available, perform similarly better).
The fact that everything gets written to the transaction log as well as the datafile is an important fact. You can speed up insert/update operations massively by having the log file(s) and data file(s) on different drives (and so on a different set of heads). You would probably be better off arranging the databases around the five drives such that each has its log and data on different drives. How you arrange this will depend on the expected activity of each database - you want to avoid two databases that will be busy at the same time being on the same drives if possible.
Another option would be to have two separate RAID1 arrays - one for all the log files and one for the data files. I suspect you will see noticeably better performance from this tan from having each DB (both data+log) on its own single drive. Or if you really don't care about redundancy (for example if your mirrors are maintained using a replication/cluster arrangement that means you'll lose very few transactions if the master DBs die) you could have a pair of RAID0s instead for a bit of extra speed.
If a little money for three drives is not a problem and your machine has room for more drives, you could have a pair of RAID10 arrays (one for data, one for logs, 8 drives in total) then you have your redundancy without the RAID5 write performance problem, you have the write performance boost from having the logs and data on separate drives, and the potential read+write boost often seen with RAID10 (4-drive RAID10 with a good controller tends to perform similarly to 2-drive RAID0). Some of our production databases are on such an arrangement and I can tell you that they fly along compared to other arrangements I've tested. If you have a hardware controller supporting RAID1E (it goes by different names on different controllers, unfortunately) then you could do the same with two 3 drive RAID1E arrays, needing only 6 drives.