I will usually build my own server for anything i manage directly. For anything that i am not likely to be around for i will generally use a dell server.
I like building my own because i can get very granular with my hardware needs and concerns and make the choices that fit the needs best (space, performance, sound, purpose, cost, etc) but if it is something that may become a support issue ever i will choose something with a very reliable support contract. Hp has never been that for me, Dell has.