Definitely go with Dell, they have great Tech support, and they're cheaper. I'm a IT Director, and we've been buying company computers from them for 4 years. They've given us great support, good prices, and we'll definitely be sticking with them for the long run. The last company I was with had an agreement with Gateway, and they used to be good, but lately they've been heading downhill. My friends at the company tell me they have become less satisfied in recent years with Gateway's support and just their products in general.
However, just to be fair, everyone's experience may be different, and you should go with what you've had the best experience with. Just take into account prices, quality, ffeatures, and support.