Well first look at the costs of each and then the service provided by each company for their product if and when something goes wrong. What you actually need is one that can be replaced within a few hours or less of it breaking is repairable rather than just binned and a new one supplied while UG.
I have a preference for IBM but that is purely a personal thing just as I always get better service from them. HP I've found wanting and I've never personally touched anything with the Dell logo on it but again that is a personal thing you may have different ideas and needs.
Col