Details on the model of Mac you got rid of, the cost for the new Dells, and the usability that you weren't getting from the Mac that the Dells offer. If the answer is "Games", then that one word answer will be sufficient. If that's not the answer, then I'm curious what it is.
That would about sum it up for me. If I could find the realistic flight sims and the military strategy games I'm used to running on WinXP I'd consider switching to Mac. Back in the late 80s-early 90s that would have been possible. Not anymore.