Hi mandrillUK,
A lot depends on what you want to do with Trainz. TS2009 has more built in content, which is good if you want to build layouts, and it is also very stable once you get it sorted. Ts2010 is more for someone who wants to run driving sessions and requires quite a lot of downloading if you want to expand it. there are also one or two bugs still in 2010 but there is another service pack to come I think. Most items that work in 2009 will aslo work in 2010 but items built specifically for 2010 will not always work in 2009 so you could update to 2010 later if you want to.
Cheers,
Bill69