As with all things, “It depends”. For such a smaller number of hosts talking to a single array, your performance isn’t likely to be constrained by your transport – iSCSI or FC. Your performance is going to be constrained by the number of disk spindles.
As far as the two technologies go, unless you’re doing OLTP, iSCSI won’t add too much overhead. If you are, you’ll find that the network overhead might not be to your liking.
I think you should compare the cost of the two back-end systems when configured with a similar number of disks and cache, then compare the cost of infrastracture needed to enable the two – new FC or GigE switches needed, HBAs, etc.
There is a difference between the performance on the two protocols. However, whether that difference is enough to affect you is a nother question. Only you can answer that by actually testing it out.