I play both, swamp thing IMO is more fun to play, but he requires patience, the ability to read your opponent, and some tight execution.
If you have decent execution though, scarecrow gets huge damage, excellent pressure strings through exdb2, command grab shenanigans up close, and better mix ups. He also has a wake up now, db3.
Both have a hard time against zoners, but IMO scarecrow has a slight edge on swampy vs zoners through his teleport when used defensively (teleport and immediately block low)... Buuut a player who knows the teleport is coming is going to bend you over lol.
TL;DR- Scarecrow until swampy gets some fixes.