Before building my own, I had experiences with both dell and gateway, and I personally think gateway is 10x better than dell. Gateway's tech support is superb, they actually seem to know what they're talking about, and whenever some hardware goes wrong, they send out a new one no questions asked.
Dell always gave me the runaround, and always responded with the "reformat and reinstall."
So, I personally would pony up a bit more money for the gateway just for the peace of mind.