RAID 1 question

I am buying a Barracuda ES.2 250gb(32mb Cache) and wanted to add another 250gb Barracuda 7200.10(16mb Cache) in Raid1. I have never used a Raid config and didn't know if it was ok that these drives have different cache sizes and slightly different read/write times. Will the slower backup drive affect the performance of the main ES.2 drive or will it simply serve as a good backup for my drive? I could just buy 2 ES.2 drives but they are about $30 more expensive and I was looking to save a little cash

The RAID will run the same speed as the slowest drive. Basically, the faster drive will "wait" for the slower.

Also, RAID is not a backup; it is if you have hardware failure, you still have the most recent copy of your data. Delete a file that you needed? Gone on both drives.