Just like yourself i have TS10 & TS12,u can merge the two together so it gives a better experience,one thing i have noticed is that it gives you better frame rates as i have noticed....
TS12 has multiplayer, a built in volume control, and doppler, does a better job of processing transparencies. Essentially it's TS2009 service pack 11 with different content. Testing with the same routes, sessions, trains, and camera angles with the exact same video and quality settings I get zero difference in framerates between TS2009 SP4 and any version of TS2010 or TS12.
Just like yourself i have TS10 & TS12,u can merge the two together so it gives a better experience,one thing i have noticed is that it gives you better frame rates as i have noticed....
TS12 has the ability to have 2010 merged with it. You copy the 2010 JA files to the same place in TS12, add the Registration key to TS12 the same as you would for an add-on and let it run a database repair. This only works for 2010 not any of the older versions. Not tried it myself as I had no desire to fill up TS12 with a load of routes and stuff I don't use. Probably safer as I did to export any routes 2010 routes you want to cdp and import them using TS12's content manager, the dependencies but not the routes for 2010 are on the DLS for TS12 users only.