They're both enjoyable series.
If you like Dragon Ball Z, Bleach, Naruto, Yu Yu Hakusho, One Piece (or other shonen anime) you'll like Blue Exorcist.
I would suggest any potential fans to take advantage of the Toonami broadcast if you're not crazy about paying for Aniplex USA's expensive prices for their DVDs and Blu-rays. Blue Exorcist is also on Viz Media' Neon Alley network (monthly subscription) but some titles don't have a "catch up" feature so you'd be stuck waiting for a rerun to stream at specific times...
Black Lagoon is for a more older audience. Expect lots of violence, some comedy and swearing. Because it's a broadcast all swearing will most likely be censored (bleeped) but the DVDs/Blu-rays and Funimation.com streams are uncensored (nobody needs to start complaining that "they ruined my ANIMU!")
If you like anime like Hellsing Ultimate, Jormungand or Trigun you would probably enjoy Black Lagoon.