Cubiyo

I've played both, and both have their pros and cons.

OpenTTD over Simutrans:

• Easy to mod: thousands of mods ("NewGRFs") are freely available. Many of them are marvelous and can expand game possibilities by quite a lot. I don't know about modding in Simutrans, but as far as I know, it is not so easy.
• No bugs (at least not as much as in Simutrans)
• Smooth multiplayer mode

Simutrans over OpenTTD

• Mainly as the map is coded by layers, the building possibilities are higher, allowing for non-straight bridges, fancy tunnels, and stations on both (metro lines!)
• Different types of roads, diagonal roads, private roads, and best of all, public stations, including public airports (however this is not very enjoyable as the multiplayer mode is laggy as hell AFAIK)
• Customizable airports; in OpenTTD there are only a few sizes of predefined airports, but you can build them as you want in Simutrans.

I personally have always preferred OpenTTD, as I like its gameplay better, especially since it included cargo and passenger distribution (where Simutrans had a huge point). However, I still like Simutrans, and now that you mention it, I will give it a try soon to see if they have fixed the cons.