1st off, ask yourself... are you really ready for a snake big enough to kill you? Both burms and retics will get big enough to take your life. Now that doesn't mean either species is aggressive at all, just means they are capable. All it takes is one wrong move. 2nd... neither is going to do well in a "tank". Hold off on any more advanced species until you're past the tank phase. And remember... NOTHING over 10 foot is to be handled alone... the last thing we need with all the fire we're getting from poiticians and the media is for someone else to get killed or hurt by a large constrictor giving them more ammunition to use to get our pets banned.
That being said. They're completely different animals. The only thing they have in common really is feed response, and the fact that they both get huge. Burms are alot less active, and generally (but not always) alot more laid back outside of the cage. They don't really seem to care which tree they're stuck in. But they're alot thicker and heavier than a retic. They're awful garbage disposals. Anything that doesn't get eaten in my snake room, my burms will take without hesitation. Though since the only other animals down there are retics it's not so often that someone refuses food. Retics on the other hand... while not as thick and heavy, definately seem to be alot stronger and faster. More agile. And their feed response is off the charts. It's not unusualy at all for one to shoot out of the cage and grab the rabbit before I can get it into the cage to offer it to them, leaving me to push a 16 foot snake wrapped around an 8 pound rabbit back up into the cage... lol.
I love my retics more than anything. But I'm not your normal reptile keeper, they're just not for everyone. Though the horror stories you hear that all retics are aggressive and mean is rubbish. Just look through some of the pics I've posted of mine. But I'll always have a soft spot for burms so I've always gotta keep 1 or 2 around. Tara hit the nail on the head when she said my big male (lil D) reminds you of a big dumb teddy bear.... lol.
Again. Both snakes get big enough to do serious damage, especially if you don't have any experience with them. Take your time, get around some PROPERLY cared for burms and retics to get some experience under your belt, and go from there. I had been keeping reptiles for 11 years before I got my first burm. And 21 years before I got my first retic. But I started at a very young age.