Well, in actual application and performance, I believe the IBMs do win, however, not everyone can afford those and if the budget is just not there, then a Dell cluster solution can satisfy some peoples needs.
I always enjoy these debates because there are so many different angles that you can approach this from. Such as:
Who has the best hardware/software?
Who provides the best support for their hardware/software?
Who is the most affordable?
Which system will be the easiest to implement in a cluster solution?
This is a loaded question and is the beginning of an excellent debate.
I will be very interested to see others thoughts on this matter as well.