Both done correctly can be difficult to train. However out of the 2 reining will take a lot longer and you really need to have a pro reining trainer to give you a hand or train your horse. There is so much more to it then what it may look like.
Barrel racing. Not to knock barrel racers but anyone with a little knowledge can train up a decent barrel horse in a rather short amount of time. Reining takes a lot more effort and intelligence. You would definitely need a REAL trainer to train you and your horse. It's SUPER easy to mess one up. They also have to have some kind of natural ability to rein.
Barrel racing shows are cheaper, the classes are inexpensive and that's all you pay for. Reining classes are more expensive and the shows are few and far between, so you will be looking at trailering, hotel, stall, etc.
Barrel racing would probably be a better bet if your on a budget. Posted via Mobile Device