This is something everyone has major opinions on on the internet, and in fact, may throw their allegiance to different sites entirely. I just happen to be affiliated mainly with those two, so I know more about them in general. However, if you expect me to be choosing one, I'm going to direct your attention back to the
Old El Paso TV Commercial.
[Yeah, I'm going to get judged heavily for that one.]
That's the thing, though. Both sites hate each other's guts, though they both also consider each other minor villains compared to the horrors of 4chan and deviantart.
It goes like this: Reddit is convinced that tumblr is a bunch of teenage girls writing bad fanfiction, making crappy social justice blogs, and obsessing over doctor who.
Tumblr is convinced that reddit is a bunch of twenty-something men making stupid jokes, complaining about how horrible their white, twenty-something lives are, and obsessing over mindcraft.
They're both kind of right. Reddit's home to a lot of jerks, and tumblr's home to a lot of crazy feminist jerks. If you weren't expecting unsavory people on the internet, you, well...
Let's just say you should expect unsavory people on the internet. It's kind of a given.
However, I'd like to remind people before they start actually tying me to the stake: people don't notice, but both sites borrow heavily from each other. Tumblr makes it a lot easier for original content users to both display and get credit for their work and to let them distribute it quickly, whereas Reddit is simply a way to share links to a variety of other places. I hate to break it to you, redditors, but a lot of those really funny exchanges that got so many upvotes? That format's Tumblr. Sorry, Tumblr, those little facts you found so interesting? Third page of Reddit.
I ship it.