Areo glass will cause people less problems then XGL. Mainly because Microsoft's monopoly status dictates hardware design for video cards nowadays and video card companies spend millions and millions of dollars designing, testing, evaluating, and programming to make sure that their stuff works ok with Windows Vista.. were with XGL they couldn't give a sh1t less.
The advantages of XGL would be that it's probably much more flexible then Areo (or Aqua). End users would get a chance to design and program their own visual effects and special 3d tricks. I figure that with enough people playing around with eye candy somebody will actually create something usefull for the Linux 3d desktop, rather then just having 'bling'.
Also, as always, X has the advantage of not being restricted to local applications, and XGL will be no exception. The ability to have indirect acceleration for vector graphics and other items while doing compositing will be a big advantage for XGL over Areo's compisition stuff. Last I noticed while in compisition mode there is no hardware acceleration for the win32 GDI stuff. Not that a lot of people will notice this since computers are so fast compared to what they used to be (and X render stuff doesn't realy do a good job of utilizing acceleration anyways)
As for 'XGL' being unstable or whatnot, it's not nessicarially true. AIGLX (which has XGL at it's core) impliments features like accelerated indirect rendering and is included by default with the latest release of Xorg's X server. (X.org release 7.1) You can use compiz with that to get Novell's special effects like cubed desktop/fancy window switching/wobbly windows, if you like.. I beleive. I beleive that it's not enabled by default though. Not all cards/video drivers will support it... Although eventually even old machines should be able to run it well.
With some work you can probably either use AIGLX + metacity's special upgrades (the luminocity stuff) or Novell's compiz on Fedora Core 5 currently. I am not sure though since I haven't tried it out yet.
As far as the resource usages go Apple is clearly the winner. Their original Aqua compisitioning stuff ran with NO hardware acceleration what-so-ever with the early releases of MacOS 10, and it was still fast enough to be usable on 300mhz machines with 128 megs of RAM.