Would there be a way to hook it all into one map? If there would, that would be a great way of getting rid of lag. Ex. Half of players would be connected to one server while the other half would be connected to the other with the map, chat, and plugins linked.
For just 5-10 players I would go for the Xeon. If you ever plan on making your server public, I would go with the i5 and install TickThreading. Since the 1Gbit port, 16Gb RAM, and more storage would help.
There are way too many griefprevention servers out there. Personally I like Towny alot better because it give it more roleplay/realism than just claiming your land and living by yourself.
That would be nice; however, I don't think it would work out well. TT is designed to work with the specific mods of FTB while MCPC+ is for any server with forge. Plus, it requires a patcher to run... :/