Sounds like everything has mostly been said, although I'd also say look at the software your using and make sure its not platform specific only. Personally I've used mac's in the past and loved them, however I wasn't so in love with the price. And from what I've read in the reviews, in the PC platform the XPS one has gotten green reviews across the board, even reviewers that favor mac systems have had nothing but good things to say about that system.
In the end go check out a mac system and see if you like it, check out the requirements of any software you may have and make sure you can use it on a mac. You have picked two very good choices in systems, but its up to you to make the final decision. All the fighting over which platform is better will never help you out.