The maemo (version 5) that comes with N900 by default is most obviously the best supported version for the device itself.

Meego and nitdroid are hacks to make it work on N900 and they both have their own issues with having no full support of the device compared to maemo nor are they recommended for beginners.

What you need to realise that N900 is not exactly a phone, it has some features of what a smartphone would have but its not a phone. For instance, there's no MMS functionality by default, the portrait mode works to some extent (with a hack and applying unofficial/community made updates), the device has a few known hardware flaws, etc.

The N900 would not suit you if you're not familiar to linux environment and/or not willing to learn and be familiar with linux environment as by default the genuine N900 is powered by linux. It is great for as an internet tablet (like its predecessors which never had phone functionality let alone the ability for one to insert SIM card and receive calls, etc) it is subpar in phone operation (as Nokia seems like they have rushed this device out onto the market with high expectations and got the opposite of what they hoped).

As I've said before on a few other threads to keen/new owners of N900 that it is a geek's toy. Everyone has their own opinion of what a geek's toy is but in my case it allows one to tinker the toy to their hearts content knowing full well that the toy may not operate properly in the process of tinkering but its all for the sake of learning. N900 is not really suitable for you if you do expect it to function as a full blown smartphone and should maybe have a look at N9 instead.

Aboveall N900 is a great device if you choose to use it after you have done your homework on what its flaws as well as limitations are and have chosen to live with it due to its `half baked' design. This is coming from a now proud owner of 3xN900 whom has two with SIM cards and is used at times as a smartphone.