For a first large snake if you havent really worked with them, a male burm will be the easiest typically. And if that time comes where you need to rehome it, people tend to be more willing to get a large burm than a large retic, just because of a retics bad rep.
Personally I prefer retics. They just provide a little more of a challenge and are a little more interesting to study behavior. When you work with snakes all day every day, you really notice some big differences in how species react to different things, and retics are my favorite just because of how aware they are of everything around them.
But typically, baby retics will give you a little more trouble in the beginning when handling. I'm not saying that burms are the "gentle giants" but they tend to be a little easier to handle and slower moving, most retics even as adults dont like to sit still when being handled.
I bring my burms to photo shoots because they will actually sit still for two seconds, you dont see a lot of retics in photo shoots typically. I've done it, but the model has to be used to handling a snake or at least comfortable with it constantly moving.