It depends entirely on your SAN.
Microsoft don't document specific SAN configurations.
So, your SAN boys are so unsure about the resilience of the disk
subsystem that they're proposing RAID10?
You need a decent SAN.

You'll need to get that info from your SAN vendor. They should also be able
to provide you with the tools needed to corectly measure IO & perfromance if
you choose to test configs yourself.. SAN's, vendor to vendor, are different
beasts ...

In "Optimizing Storage for Exchange Server 2003", page 36  "In general,
Raid-5 does not provide the best trade-off between reliability/availability
and performance."

http://www.microsoft.com/downloads/details.aspx?familyid=c6084d20-9730-4ffc-805d
-b957327604c6&displaylang=en

Raid 5 is generally not recommended because the write penalty is too high.
You'll use far more spindles to reach a given performance level using RAID 5
than you would with RAID 10.

Here's a link with recommendation for raid levels.

http://www.msexchange.org/tutorials/Optimize-Exchange-2003-Performance-Part1.html