The recent builds of Galacticraft that just came out is intended for 1.6. Although, I wouldn't recommend using those for anything but testing purpose.
I did the mistake of updating Minecraft to 1.5.2 right away. It wasn't worth it. It's not only because of Galacticraft needed to be updated, but all the other mods as well.
Except from the ton of bug fixes and optimizations, I don't think there is much 1.6 has to offer (but then I don't play vanilla Minecraft either). Who'd need horses when you have rockets that can take you all the way to the moon? Well, perhaps space suits for horses could change that. Then one could play as a space cowboy. Although, even then the moon buggy would still be preferable.
Today I had to get through forests and deserts, from a horse spawn area 2 km away from my base (I still play a beta 1.7.4 world). I made my way back home very quick with a horse, I don't know if I could do the same thing with rockets.