I've got both a G5 and a G9.
Both mice are superb, with the G5 probably edging it due to it's (imo) superior comfort and better weighting system.
I've only had the G9 a short while and haven't really used it for any lengthy gaming sessions yet, but it is as people above say; a claw grip. With the G9 your palm doesn't really touch the mouse, only your finger tips, with the G5 your palm rests on the back of the mouse.
Both mice have a nice feel, both mice have the braided cable (which no doubt is more durable than the plastic cable) and both mice glide effortlessly over my Razer eXactMat.
The G9 uses different Setpoint software, which in all honesty is pretty decent so far with loads of additional settings to play around with inc profile memory, macro facility, changeable LED colour (which seems more than just a gimmick).
The G9 comes with two different interchangeable grips, one being rough like the surface of the G5 and the other being sort of rubbery smooth. Logitech have apparently said that they're planning on releasing a few different grips for the G9 so I'll have a look out for those. At the moment though I think I prefer the rough grip out of the two.
If I could have only one mouse I think I'd chose the G5, but I definitely wouldn't be disappointed if I could only have the G9 as it's a quality mouse too, just a little different to the normal Logitech design.