Each of these RAID levels offers different performance levels. Ideally, if your budget can support it, chose RAID Level 10, which offers both high-speed and fault tolerance. Very expensive, but very fast. Statistically speaking, the odds are the data SQL Server wants next from the array will already be in the cache, speeding up data access. This is because the act of caching itself takes some overhead.
Testing is the only way to know for sure.
The problem with this is that SQL Server will not know about the write caching, and will assume that when it writes a page to disk, that it was written successfully to disk. While this will be the case in most instances, it may not always be the case. If the server should fail before the data in the cache has been written to disk, then that data will be lost and your SQL Server databases may become corrupted. While a battery backup for the cache can help reduce this problem, they introduce their own problems. For example, what if the server's motherboard fails and you need to move the drives to a new server.