The only advantage to using Enterprise is it supports clustering and more processors/ram. Windows clustering doesn't mean anything to PSA, so its a good waste of money IMHO.
We use Web Ed for our servers. While you can't install Exchange or SQL on Web Ed, if at some point we needed SQL server, we would load a new server specifically for running SQL. If your to the point where you need SQL, you don't want to run anything else on the same server with it. Until then, DE and MySQL are enough for most.
Aside from not running large enterprise apps like SQL or Exchange, Web Ed is the same as Standard.
****NOTE****
If your going to use Windows for hosting, the OEM EULA that comes with the cd does not apply to you and is void by your environment (reselling windows services for profit). You have to contact MS and get Service Provider licensing. This also goes for buying new servers with Windows (i.e. Dell, HP, etc) as the OEM EULA is targeted for use on internal networks. AFAIK, this doesn't apply to servers you get from The Planet, ServerBeach, etc..