If it's for use in a business I would say go for the full retail version. The cost of the software is an almost negligible part of the total cost of ownership. The cost of CALS alone is going to dwarf the initial cost of the OS.
But if it's a home setup and you are fairly competent then go for the OEM version (with the restrictions mentioned above).
I forgot about the connection licenses. I don't know if the OEM version would have any built-in. I think Server comes with 5(?) connections when you purchase it, you may need to check on that for your needs.