All are good vendors but if cost is a major factor, I would recommend you lean towards Dell since they are really focus on the small and medium-size businesses unlike the other vendors. All the vendors will give you everything you want out of performance and features.
Dell is the lower cost at first blush, but you can get others to come down to acceptable levels.
I like IBM’s blades. They have had a long run, and are backward compatible. Dell at this point is still trying to figure out where they are headed, and comes out with a new model that is incompatible with older ones fairly regularly.