Though if you're following gaming news, there's a claim from a Newspaper that the 360 just recently got hacked. Microsoft is denying the claims and to some degree it seems like the Newspaper is over stating hacking, when it is actually phishing, but either way. The 360 is no more ignored by people looking to steal or exploit others than the PS3.

But I would say you will probably want the largest HDD you can get for the PS3 or get the cheaper one and immediately upgrade it to a larger one. I'm planning at some point to get a 500 gb for mine because the 40gig I have is just too small. I have to delete game data a lot because it adds. I do have one game consuming 20 gigs of it, but even still it adds up fast.

I'm not even a person that plays a ton of games, but I find the 40 to be too small and I'd probably find the 120 too small as well. Because if you do the install on your games it will end up filing things up quickly. If you replace your HDD early you won't have to worry about the backing up of data. The only hassle will be burning the firmware to a CD to get the new HDD with the fireware. Sony explains all of the steps and the backing up process is probably the longest. Replacing HDDs is really fast.