I've dreaded this for many years. Now, the day is upon me.
Despite DC's iconic leanings and the assumption by many that "DC did it first", Aquaman is not Namor's predecessor. It was in fact Namor who appeared first (in 1939) and there is a case to be made that Aquaman is actually a copy of his Marvel counterpart. (Aquaman first appeared in 1941 and was supposedly based on a character from a sci-fi book.)
Okay, hold up. I just erased an entire paragraph describing Namor the Sub-Mariner. Why? Because I wrote it, and then realized these guys are so similar, that it would be boring and pointless to write a similar paragraph about Aquaman. Sure, they've had lots of different crap happen to them and at their cores they aren't the same man, but in trying to describe them peripherally - unless you're a hardcore fanboy - they're pretty much the same.
Shit that's the same: Human dad + Atlantean mom. Both royalty and lead Atlantis. Known for being hard-asses as well as good guys. Powers almost identical (breathe underwater, enhanced vision, superstrength, weakness to being out of the water).
It's the stuff that's different that really needs to be focused on. Namor is Marvel's first real anti-hero. He's just as likely to be attacking the surface world as saving it, whereas Aquaman, though annoyed at the surface on occasion, is mostly a good guy who'll do what's right, not just what he deems most benefits him or his people. Namor can fly 'cause he's got wings on his ankles. Aquaman can talk to the creatures of the deep and ask them for help, like if someone wants a whale ride.
As for the versus question, I'm not going to take that in the direction of a fight between these guys. That would be fun, but as pointless as comparing them. To me, it comes down to this. Namor is a complex, interesting character, who is as comfortable beating ass as he is dealing with the machinations of the Marvel universe. But to me, he's just another great super hero in the long line of great Marvel superheroes.
But while the Sub-Mariner comes across as a violent, tempestuous 'prince' who doesn't take any shit, Aquaman is a king. He acts like a king, performs his duties with honor and morality, and will beat your ass if you've got it coming anyway. Also, when I was a kid and I discovered Aquaman, there was so much wonder involved, because he was like other superheroes, but he lived in the ocean! Holy shit! Namor's books never felt like they were really about that. The early Namor, for example, when he was simply a bad guy for the original Human Torch, did most of his fighting on land, flying around with those damn ankle wings. Aquaman comics are often about the fact that he's the king of Atlantis. This is also probably why Namor gets a little more respect, being a 'well developed Marvel character', as opposed to Aquaman who has that 'not as well developed Iconic DC character' stigma and talks to fish.
I'm trying to be cool and unbiased, here, but to me, Aquaman has always carried a lot more weight. Namor's a cool dude, but Aquaman is King Arthur with sharks.
(I read this whole thing and added this part after. I thought about some of the concrete things, like who of the two characters has the 'best of' in certain categories. Here's what I came up with. Best Villain: Black Manta (Aquaman). Hottest Chick: Mera (Aquaman). Best Moment: When Arthur basically tells Superman and Wonder Woman to eff off in Kingdom Come when they come looking for his help (Aquaman).)
Short Answer: Bias, you say? I have a huge framed poster of Alex Ross's iconic Aquaman painting on my living room wall.