If I'm honest, the PowerMac. I don't like the build of the MacBook Pros, which is funny because I'm on my second one. But I have no other choice in Mac laptops, so I'm stuck with it. My first MBP had the paint chipping issue, the warping, two times where the SuperDrive died, two times where the battery died, one of the fans died (due to the Genius not closing my machine right when doing a repair - I bring this up EVERY TIME, LOL, but it just makes me SO mad), and I was never really happy with it.
Lion drove me crazy on the new one, I started up my machine one day in like February/March, and it decided to install a non-exsisting software update at boot. It rendered the partition useless, I lost EVERYTHING, and I had to reinstall Lion AGAIN. Oh, and when it came time to installing Mountain Lion, it corrupted my drive that day. Luckily I had backed up everything anyway. But reinstalling OS X from Apple's slow and pathetic servers is a nightmare.
Otherwise I enjoy using my MBP, and it stays in my bed 24/7, and comes with me whenever I leave the house for long periods of time. Now that I've settled into Mountain Lion, it's 100% stable.
I really like my PowerMac much more because it doesn't give me a hard time. I only have 512MB of RAM, but it does pretty good. I have never had any freeze ups or slowdowns like I have on both of my MBPs. I don't even use it casually, I push it really hard for a really old machine still, I render, I use pretty much the entire Creative Suite package, I load heavy Flash sites, and I do it all without the fans going crazy. Unlike the MBPs which get really, really, really, REALLY hot with just Safari open.