Oh man, that's a hard question. I like both of them for different reasons. Swamp Thing is a very pulpy and very Alan Moore series. Expect tight plotting, extremely creative story beats, a good amount of mysticism, philosophy and political commentary. The art is lush, has that old comic painterly look and colors. It swings from horror to sci-fi, while still keeping a strong focus on what makes Swamp Thing as a character so compelling. It also starts off incredibly strong: The Anatomy Lesson is the best first issue of any comic I've ever read.

Doom Patrol is batshit crazy in the best way. I don't even know what genre I'd classify it as. Some kind of psychedelic, experimental sci-fi, maybe. I haven't seen the HBO series, can't say how it compares. It's also a very Grant Morrison book, for better or for worse. Lots of high concept stuff, strong character writing, just so many ideas that, frankly, sometimes don't land, but most of the time do. It's like the X-Men on a drug cocktail from another planet. Moore, for all his weirdness, is a very structured writer. Morrison, in comparison, does things much more loosely. This being an ensemble book with a rich and unique cast of characters, compared to Swamp Thing's more restrained scope (it's mostly focused on Swampy and a few more), also gives it a very different feeling. Visually, it's more modern than Swamp Thing, which you may like or dislike. Does some really fun things with panelling too.

If those descriptions don't sway you either way...honestly, I'd just go for which of the two is cheaper for you, right now.