Although it really depends on what your primary needs are out of these servers, I tend to side with IBM. First of all, their support model is much better and their consulting services that they provide along with the blades (for a fee of course) is much better than HPs, which is almost non-existent. Maybe this acquisition of EDS will change that.

Also, if you have legacy hardware of either type, you definitely want to go with IBM. They have been successful in making their chassis totally backward compatible with their older modules and blades and most of their newer modules and blades fit in their older chassis.