In this case, prime, there are three factors you want to consider: expandability, size of screen, and ease of use.
(NOTE: When I'm talking about the iMac here, I'm assuming you want to get the high end one with the SuperDrive and the 800 MHz G4 processor, because IMHO that's the iMac with the best value. Currently, it's priced at $1899 with the price increase that was recently announced at MacWorld Tokyo.)
If you think you'll probably want to expand your computer using INTERNAL PCI components and/or an internal zip/optical drive, then you'll probably want the PowerMac just based on this fact. Note that with the iMac, you can still add external devices galore, like FireWire hard drives, CD burners, and TV adapters. For me, this isn't too much of a problem. The iMac, though, will be a pain to upgrade the graphics card (which is probably not even possible -- only unsupported hacks will work, and even then you never know) and the hard drive and the optical drive. The iMac is basically geared to not be upgraded except externally (with the exception of the Airport card and one memory slot, which are incredibly easy to install).
Also, if you want more than 15" of screen real estate, you'd also probably be better off getting the PowerMac and get any number of larger CRTs are LCD displays. 15" is starting to get a little cramped for me on my cube, so I'm debating whether it's better to get a PowerMac or an iMac (whenever I decide to get another comp).
The last thing you want to consider is ease-of-use. The iMac is SOOOOO much more easy to manage, not to mention it has an awesome adjustable screen (I actually think that the adjustability makes up for the relatively small screen). Do you want to have a big screen or expandibility, sacrificing other factors by having a big machine that hums loudly under your desk and is a mess to get to the back? Or do you want to have a nice compact computer that can sit on your desk and take up not so much space, at the expense of expandibility and/or a large screen?
Because you've said that this is probably a 733 MHz PowerMac, it'll come with a GeForce 2MX card, which is the same as the iMac. So you're basically getting a little faster power with the iMac here, but it's probably not going to be noticeable.
Personally, I'd go with the iMac, but it's your choice.