These are pretty much the same experience, with slightly different theming. There really isn't much to differentiate them so go with whichever theme works for you. Or just flip a coin.
They are basically the same, as a PP said, just different theming. I'd say T-Rex is slightly louder though, but maybe that's because I haven't been to RFC in a few years & I'm not remembering.
We went at the end of January this year. It is very, very loud. I'm trying to think of something to compare it to if you've never been to RFC, but I'm having a brain fart at the moment. About every 30 minutes there is a "meteor shower", and all the dinosaurs start making noise and it gets very loud. It could be scary, but only you know your child.