Hooks are usually easier to get, but also easier to lose. This also makes it easier to switch to armbars.
Body triangle can be harder to get (though I find it easier from some angles) and is harder to escape. It can make leaving rear mount for other moves difficult. You need a good RNC to make it really worthwhile, unless you like to just hold rear mount forever but not do anything.
Body triangle can also be bad for your knees.
Roy Harris answers a Q&A about this on his site.