my 2 cents:
OTTD>Simutrans
+ metric f*ckton of mods for OTTD, starting from new graphics and ending with various patches. Simutrans has the Experimental beta and a couple of graphic sets and thats about it.
+ easier learning curve for OTTD (and not just economy - also UI, signalling etc.. - both of those are a mess in ST IMO, and outdated or missing documentation doesn't help either)
+ Simutrans is WAY too cavalier with system resources usage for my liking..
OTTD~Simutrans
~ road vehicles in ST are handled similarly to trains - offers a lot of flexibility (trucks + various trailers, trams etc..), OTOH also causes some issues (only 1 vehicle per direction and block etc..)
OTTD<Simutrans
- Building options (underground, bridges, airports) are limited in OTTD