As I understand it:
rev13a included some change that required a small change be made to loaders for them to work. NeoGamma is the only loader I know for sure has the fix, but I haven't been following the threads for all the various loaders out there. At any rate rev13b was released so that older loaders would work with it, but supposedly load times will be longer.
The upshot, use rev13a, but make sure the loader your using is compatible with it. The required change was very slight and doesn't hurt backwards compatibility, so any loader being actively developed should have the fix already.
rev13a included some change that required a small change be made to loaders for them to work. NeoGamma is the only loader I know for sure has the fix, but I haven't been following the threads for all the various loaders out there. At any rate rev13b was released so that older loaders would work with it, but supposedly load times will be longer.
The upshot, use rev13a, but make sure the loader your using is compatible with it. The required change was very slight and doesn't hurt backwards compatibility, so any loader being actively developed should have the fix already.