There are a lot of great replies to your question already here, but I thought I'd add some additional anecdotal information to consider.
I have owned around 7 portables over the years and the single best I've ever owned has been my current Mac Book Pro. It's simply the highest quality portable that I've ever had. The second portable maker that I've loved has been Toshiba (having personally owned 2 and purchased 5 for staff). I did like the power of the old portable Dell I had 10 years ago, but it was heavy (I needed the demo speed) and really unfun to lug around. I have a Vostro class desktop at work and I don't like it much. 2 things aren't working on it (usb/line-in ports), the drive bay doors are downright stupid, it's badly designed, and cheaply made. Sure they're inexpensive, but if you're going to travel with it, I'd make sure that those Dells will survive the trip. Generally, I love Dells having purchased around 30 of them over the years (higher end models) and have had very little problems with them. I'm just not so sure about the Vostro line of Dells. If you haven't already I'd see what current customers are saying about them.
If you're moderately technically inclined you can set up the MacBookPro to dual boot, and or run windows virtually. I've been testing VMWare Fusion and it's really nice. SketchUp is pretty darn fast in a virtual window - and I can barely tell the difference between running SU virtually or running the Mac version while in Leopard. Plus, if need be, you can just boot the machine as a windows machine (I use XP).
In general, the reason I'm doing the switch is because I'm using more and more of Final Cut and Motion. I guess you have to ask yourself what software you currently own, and what direction you want to go and see where the financials come out at the end. With dual booting/or virtual windows you can still use windows based applications. But if you're not going to use the advantages that a mac might offer, you might be better sticking with windows. Juju brings up a good point that learning the Mac OS will require some ramp up time - it did for me - though I was up and going pretty quickly. I also like the benefit of knowing both of the OS' as I do more video/film presentation work and it has broadened my skill set considerably.