I would go with the J-3 Cub (I own one).
The Decathlon is basically the same plane, but a little heavier, which is not as good since the J-3 is not a powerhouse to start with.
Using a different prop on the J-3 gives you more power and speed (less crashes from stalls as you are learning).
There are lots of recommended upgrades on both of these planes in this forum. I fly my J-3 almost every day.

Lots of luck!

Jorge