There's not a whole lot of difference between the two, but if you're new to management, I find Simutrans to be easier to get a grasp of and start working on.
I think the main difference between the two is the fact that you can run different packs of trains and vehicles for each different game you play, with each of them offering a different set of challenges and foibles. As far as I'm aware in TTD, the only vehicle set is the one you get at the start. The pack I use is the UK pack, and it is loaded with English, Welsh and Scottish trains from way back in the 19th Century to the modern and efficient electrics and bi-modes in operation today.
The only major problem is the lack of a tutorial feature - you're left to either figure things out for yourself or find a video on YT. But it just adds to the challenge >:)