I've been using 64-bit 1.8.0_x since it was released before 1.7 packs existed, and seem to have lucked out by not encountering any of the issues with 1.6 packs. I'm currently using 1.8.0_25 because I've not had any problems with it and have seen no need to update.
A few rules of thumb:
1). If you will only be using 1.7 packs and newer use Java 1.8. The new default ram settings should need no additional modifications.
2). If you will continue using 1.6 or older packs use Java 1.7 as there were a few issues with that version of Forge and Java 1.8. With Java 1.7 you should set your command switches to Minimum Ram=2GB, Maximum Ram=4GB and PermGen=256MB to avoid most memory issues, as these are the new defaults with Java 1.8.
Java 1.8.0_25 is the supported Java version for Vanilla MC 1.8, and can in fact be dynamically loaded by the new Mojang launcher, so you don't have to keep upgrading Java to play Vanilla, and Mojang will keep the "approved" Java version updated as needed.