I have always gone with lexmark... supposedly HP is more reliable, which I believe. I guess the difference is similar to that of a Chrysler to a Toyota
I've used both and I have to say that's HP is much easier to configure, understand, and work with. HP is also a lot easier to work with when it comes to warranty issues, so if you have a problem, you don't have to worry too much about getting it fixed/replaced....
What you need to look at is the price of the ink cartridges. Trust me, when you add it up over time, your ink ends up costing WAY more then your printer, so go with the one that has the cheapest ink cartridges. I believe that is HP, but i'd have to double check and I don't want to cause I already have a printer ;P
If just between HP and lexmark, then definitely HP. HP usually cost a little more, but it is worth it in the long run!!
I have a Lexmark printer and the cartridges are expensive. And I don't know if all Lexmark printers are like this, but with mine, I don't even use up all the ink in the cartridges before it starts printing faded words on the page (if it'll print them at all). And it gets the colors wrong when I'm printing anything with pictures on it. I'm planning on getting another brand when I get a new printer, so I wouldn't recommend Lexmark either.