You want to look at the RAID, but also the storage system it's being implemented on. For example, what is involved...
in migrating a LUN or a volume from one RAID level to another, and can it be done by the controller while data is actually being read or written. Many vendors tell me, yes the data can be accessed while it's being modified, but mumble and answer when pressed about the 'read and written' part.
Regardless of the RAID level, how is it optimized for sequential IOs versus random IOs -- both reads and writes. Keep focused on what level of service you need for your various applications, and why you are using a particular RAID level to meet those requirements. Is it cost savings, to reduce the number of drives or power consumption? Or, is your requirement for more performance? Are you looking at dollar per capacity or dollar per watt, or are you looking at IOPS per watt? Look at how cache is implemented and used in conjunction with the RAID controller. And pay attention when you are looking at solid-state memory to whether you are using NAND flash or DDR RAM. DDR RAM is going to be more expensive, but you get what you pay for. It's different levels of service for different workload needs.
Check out the entire RAID FAQ guide.