It really depends. Custom builds are better cause you can use a high-end motherboard and they are upgradable. You can choose your stuff. You can't choose with Dell. But on the other hadn, if you don't want to build your own, Dell's are easier to install. There are also many companies that build computers where u can choose the parts.
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....