The extra hard disk is just more memory to allow for more storage of info -game save data etc.
It’s not worth the extra cost initially so don’t worry about that.
I’d say the opposite. Adding a disk afterwards is more expensive overall than getting it at the outset, and it’s pretty much mandatory if you’re going to be using the console online. There’s lots of free stuff to be had, demos and the like. Don’t the the 4Gb version, you’ll regret it.
XBox is cheaper yes but, remember that to play online you need to keep paying monthly/yearly or whatever
PS3 is free
Conversely, the online experience on the Xbox is far better. Given the choice and all other things being equal, I’d rather pay £30pa for Xbox Live than have the PS3’s free offering.
PS3 imo is more versatile, mine gets used more for the media side of things than the games though.
PS3 is a Blu-ray player, which is a killer app for some people. Not sure as there’s much else it can do over the Xbox in terms of ‘versatility’ that a nine year old would be overly fussed about.