Well, it's personal choice. I have both and do development on both. My home office is a 27" iMac and a Toshiba laptop.
If you decide to get another HP, I highly recommend downloading and running "PC-Decrapifier". HP overloads its machines with crap when shipped and some of it isn't easy to uninstall (Vongo anyone?). NONE of it is necessary (and don't get me started on their printers... excellent hardware, amazingly bad software).
So what are you planning on doing with the machine? Just browsing with a web browser? For your store, do you use specific applications like QuickBooks that may not be available on the Mac?
While the cost of a Mac is higher, I can easily attest to it being because they pay attention to the small details, something people here understand. A PC generally has more software available for it. I say 'generally' because if you don't need a category of software available for Windows but not for Mac, does it really matter?
They are different so if you pick a Mac, realize the first couple days with it you'll wonder "where the $%^ is that option I always use on Windows!?"
Windows 7 is much better than Vista. They finally incorporated the best ideas of Virtual PC to solve a lot of issues.
That said, though, I always reach for my TS-75 (because I don't have a TS-55

)