You have decided but I will put in my twopennyworth anyway.
I build a fair few routes, started in 2004 and am still using that version because I find it is the most stable one available and I even bought two more copies recently as backups. I do have 2006 which is not bad, missed 09 and got 2010. I won't be buying 2012 due to it not having Compatability Mode so it's worth taking that into consideration.
There are still problems with 2012, it is 2010 updated really as far as I know but I am happy to be corrected on that, so whatever problems you encountered in '10' may well surface again.
Building in an earlier version such as 2006 if you are happy with it might make it more stable to work in and you can always Import it into a later version when you're ready to Upload.
And on the uploading thing I was under the impression that there is a limit to the file size that can be uploaded, that is the reason I build a long/large routes in sections.
I am still in 2004 and can upload to the TPR site. Then folk can download and put it into later versions, but there may be problems especially with spline objects which are set up differently in the latest version I believe, althouh TBH I have not checked as I don't upload any of my content as 2010 assets. All my own content works in versions up to and including 2010 even though they have earlier configs when I test them here. I Upload to TPR as 2004 content.
I expect people will jump all over me for this post but I work in a version I know works without any hiccups. Tried both 2006 and 2010 but went back to '04', which must say something.
In the event every version will lose support after about 4 years so I don't take that into consideration. All the stuff from way back when is still available and more is still being made.
Good luck with the route whatever it is and have a lot of fun, that's the whole purpose of Trainz.
Angela