I've used Mammoth P but not the Overgrow, but the Overgrow is ridiculously cheaper than the Mammoth based on price alone (and if it comes with good reviews, you might have answer right there.) I have no idea what's in the mammoth p that they think is worth so much money in my opinion, pretty sure they milk a leprechaun to make the stuff. The folks at Mammoth are great to talk to though, we've considered seeing if they want to come on for a test trial on the site. They used to run a free trial on the site where they would send you a massive bottle of the stuff and two smaller bottles for free (you sometimes can e-mail them directly even if the promo is over or says it's ended and they will STILL send you samples.)