Actually the interface of OpenTTD is also not very user friendly from case of UI design. Like people do not know about button help text, same functionality with different text etc. OpenTTD is less straight forward than simutrans, if you never ever played a transport simulator. (On the other hand there is much more manuals around, maybe due to this ... )
Something offtopic: While Google trends hat about a 5/3 ratio of OpenTTD to Simutrans in 2009 now Simutrans does not generate statistics at all?!?!
I like to look at great maps and see how things flow. A little like a finished model railway, but it is evolving and actually never finished. http://www.simutrans.com
The biggest problem of Simutrans is that almost all of it's graphics packs look very unprofessional. They feel cold and detached, most common mistake is plastic looking ground tiles. Also a lot of them have inconsistency problems. Actually same i can say to the current 32bpp OpenTTD pack, although many renders in it are seriously miles ahead of most of Simutrans graphics.
If you want an example of how graphics should be made, look at OpenGFX, it's vivid, it's consistent, it far surpasses most if not all of Simutrans packs.
This all lies in the skill to portray objects in isometric art. And in this area the Patch/OpenTTD community is far ahead, I'd also argue about quantity of graphics too. This community has made a lot of content over the years.
From what I see Simutrans has the edge in engine capabilities, but it lacks the talent to use them all.
Professional graphics: The only one are the original TTD graphics, as this were the only one paid for.
But then it really depends, since simutrans has so many different paks: Pak96.comic has a very consistent style (just still lacking airplanes at the moment). pak.german has nearly all buildings from the same person, with a very consistent look and really very good trees, giving the landscape a very consitent look. pak128.britain has everything rendered (with a style I am not too keen of, but they are consistent.)
Personally I find OpenGFX graphics to 8-bitish, I am really not very keen on the dithering. Also the lack of night mode and winter is disappointing.
If we compare standard sets (pak64 against OpenGFX): towns in OpenTTD standard set are composed of extremely few buildings, they look nearly the same all over the game. OpenTTD ground tiles are always with a grid. And the TTD trees look really like a cartoon. (I know I can fix this with newgrf. But so you can do for simutrans.)
For the large 32 bit graphics, the main problem are the short trains in OpenTTD. And the different sizes in 45° are already there in standard OpenTTD, making vehicles only look right in diagonals.
I like to look at great maps and see how things flow. A little like a finished model railway, but it is evolving and actually never finished. http://www.simutrans.com
