If it's your first time playing with linux, I'd recommend virtualbox. Then run whichever distribution you've chosen (or several) as a virtual machine within windows. Being able to save the state of the machine and revert when it goes wrong is lovely. When something breaks, you haven't just locked yourself out of a server. I've locked myself out of a computer sat on my desk, and had to pull the hard drive out and reformat to get it running again. Embarrassing.
Aside from that. they all look fairly similar over ssh. The differences lie in how frequently they update themselves, and general philosophy regarding stability vs having the latest code. Arch and gentoo are likely to change daily. Both are lovely distributions, but as the former is simplicity taken to brutal extremes and the latter involves a hell of a lot of compiling (followed by debugging, and trying to work out which errors matter), neither is a great choice here.
Debian 6.0 (Squeeze) will be unchanged for quite literally years, it's "finished" in a sense. So if you get debian working today, you can come back in a years time and it'll be running exactly as well as when you abandoned it. It also has an excellent package management system, "apt-get install git mercurial" is probably all you'd need to type to install both.
Centos / Redhat are the same source. The latter is supported by a company, the former is a group of people taking the red hat code, removing the branding pictures, and recompiling. They should behave identially.
Fedora is red hat testing / unstable. It'll have newer code, and it'll break more often.
Ubuntu is debian testing, patched by the ubuntu devs for a couple of weeks, then released. The end result should be more stable than debian's testing distribution (7.0 / wheezy) but in my experience is not. It has extensive, friendly, and (imo) rather ignorant forums.
I think Oracle is unix, but it's beyond my knowledge.
I'd still vote for either centos or debian. Personally, I'd use debian. I started on ubuntu, lost patience with it breaking all the time, and debian stable was the obvious choice. If starting afresh, I'd have gone with centos as I'd like to have some of the red hat qualifications on my cv.