Servers are built to run 24/7. They usually have redundant power supplies and more robust cooling. (I run my servers only using one of the available power supplies.) That being said, they are probably not as efficient, on average, as a desktop ( this may vary quite a bit depending on configuration.) I have run both servers and desktops 24/7 and experienced little to no problems with either.
Edit: Also, server boards will come in multiple cpu configurations - 2, 4, etc. I don't think desktop boards do the multiple cpu's.
Edit 2: Thanks, Old Chap. I should have mentioned the fans in a server can be quite loud, especially at startup or in a warm environment. They are meant to move a lot of air.
Cheers

Workstations work great. I built a dual xeon westmere system. dual hax chips. Easier to maintain. Been running over 3 years straight. Zero issues. Can't really OC them though. You need to decide what will benefit you more and how much money you want to spend. My i7 is dual purpose. Crunch and gaming.

It would be great if it wasn't that heavy! Just make sure that postage isn't equal to cost of server...it's $400 shipping cost from US to EU!

I'm running four of these, each with dual X5690s, and a DL360 G7 with dual X5660s. They are quite loud (although still not as bad as the 1U quad-socket systems) and draw a bunch of power, but they're extremely reliable and stack up quite nicely. The price is fantastic for the computing power.

Disregarding electricity, its by far the cheapest way to get maximum compute power and time per dollar. Servers tend to have many cores for multiple concurrent threads, not great for gaming, perfect for boinc projects.

That being said I haven't bought a used server yet because I have nowhere to keep it, my room is already borderline too hot during the summer, and thats turning my PC off at night.