Everyone in my family (besides me) uses DELL, they don't all like them... I have a DELL that I have (no choice) to use at work, I HATE it, and I have a friend that has owned a few Acer's, and he hated them...
I hate Dell but I hate Acer (Asser) with a passion. I'd go with a Dell pos before an Acer one. Compaq usually has pretty good prices if you're looking at getting a prebuilt system and I like them a lot better.
Dell optiplaex (opticraps) have unsafe/unreliade monitors i was shocked by a brand new dell monitor while installing it! i dint know about Acer but they are probably the same, i have a compaq and i am happy with it. if you are going prebuilt make sure you are getting SATA hard drives, PCI-e graphics and 64-bit processors, also compaq goes with AMD cpus.