I've set up both W2K3 servers from Dell and somewhat more expensive XServes for two different small cross-platform offices. Either will, basically, do everything on your list.
That said, if you have the hardware budget but not a lot of onsite IT expertise, the XServe with OSX Server is much easier to set up and administrate based on my experience. For what you're doing, it handles everything you mentioned out of the box, and while there's somewhat less handholding than 2K3 SBS does, I found it easier to get my mind around and I was MUCH happier with the Apple hardware in comparison to the Dell box.
Some of this may have changed with the new R2 release, which I haven't used at all, so take my recommendations with a grain of salt.
1) Both will do this just fine, but depending on the number of Macs on your network I've found it a little easier to deal with connecting Windows machines to a Mac server than vice versa--in the case of the latter everybody is using Windows sharing, while in the former lets Mac clients connect with AFP and Windows clients via SMB.
2) If you mean backup of data stored ON the server, W2K3 has a backup manager built in that more or less does its job. Of course, a $30 piece of software will let OSX do the same, so I don't consider that a huge advantage, but it's odd that Apple didn't build some sort of basic solution in. This is operating under the assumption that you have first-level protection from hardware failure by using a RAID mirror on the server. If you're talking about backing up stuff to the server, I don't see that it matters--it's just a fileserver either way.
...unless you want to use the synchronization features to back up the entire user environment of a local computer to the server. In that case, not surprisingly, OSX Server handles Macs better than Windows clients, while W2K3 Server handles Windows clients more easily but I don't think can do Mac clients (somebody correct me if I'm wrong).
3) Since it's a server, and it doesn't sound like one that's going to be getting hammered to the point that its processor power is an issue, I'd guess that a G5-based XServe is plenty powerful, and I see no reason to wait for new hardware. I have a single-processor G5 XServe, and it has no trouble servicing a dozen Macs and about a half dozen Windows boxes simultaneously--processor load has never been even close to an issue. If anything, I might consdier sticking with the G5 due to it being a more tested platform, especially if you can get a deal on a slightly older model. Then again, an Intel-based XServe (assuming one is released soon) would let you run Windows on it if it turned out OSX Server wasn't doing it for you.
I forget--does SBS have a limited number of client connections? I know the version of OSX Server that comes with an XServe doesn't which is rather nice.
One other thing to keep in mind: If when you say website you're talking about an external website rather than an internal busines one behind a firewall, in my experience there are a lot less security patches you need to keep up with on OSX than SBS, and the flaws are generally more benign. That's not an excuse not to keep both up to date, and you can set SBS to automatically install and reboot patches at a certain time of night, but if I were exposing the server to the raw internet, I'd fee quite a bit safer with OSX than SBS.
