The only drawbacks with the HP that the Mac doesn't suffer from are:
Vista OS
The major downside of 64-bit Vista (aside from the Windows ME-like stability) is the astonishing lack of driver support and application compatibility. I made the mistake of buying 64-bit Vista Ultimate for my DAW, thinking 'Wow...I can fit a boatload of samples into 16GB of RAM!' A short list of things that didn't work:
MOTU 896 (Turn it on and Vista blue-screens...weirdly, my HD192 works fine)
M-audio Midisport 8x8
M-audio Keystation 88 (M-audio has had 'beta' drivers forever, but not to the general public)
AKAI S5000 w/USB card
AKAI MPD16 (AKAI just doesn't do Vista, apparently)
To be fair, a lot of that stems from the fact that hardware manufacturers looked at the new Windows driver API's and collectively said 'No thanks Bill, we'll just wait until Windows 7 comes out'. Or, in the case of AKAI, declared 'No new drivers for you!' The same sort of thing happened back when Windows 2000 came out...it took a good two years for the bulk of HW vendors to ship drivers, and by then XP was out.
Apart from the driver woes, I've had CPU and disk usage spikes for no reason in the middle of a session that made Cakewalk drop out (on a quad-core monster with an 8-drive 3Ware RAID array, mind you), a litany of blue screens while setting the machine up initially, inexplicable crackling from 32 bit applications (iTunes, how I miss thee), random Explorer crashes...you name it, I've seen it break. If you can get the downgrade to XP, take it and save your sanity. Vista is hardly usable in its current state. Some people have luck with it, but I'd rather not play the digital equivalent of Russian Roulette every time I buy or build a system.