To me, Merriman is the kind of player that is great when given a narrow focus, let him run downhill and he will knock the snot out of you, very intense player similar to maybe a Arrington or a Trotter. I think his career will be similar and that his lack of smarts and agility will limit him in schemes, especially in coverage. Very strong, intense player that will get out of position at times, trying to make the big play, instead of being disciplined. I don't think he will ever reach the level of Peppers, Kearse(until recently) or Julianne Peterson in terms of freakish agility, strength and playmaker ability but I guess you never know.
Ware, so far has been very solid and is using his hands well to get off of blocks on run plays as well as applying pressure on passing downs. Very few if any times, do you see him out of position or getting pushed back. His progress seems to be steady each game. He hasn't dominated a game yet, but he shows up on almost every play with a positive outcome... by this I mean, he wins his 1-on-1 90% of the time, whether that means pushing his guy back, drawing a chip, holding his ground on a run play, etc... he appears to be doing his job and it's allowing others to make more plays as a result.
I think most coaches will tell you that they look for consistancy in players and that has been what Ware has been so far. He appears to be fundamentally sound and making very few mental mistakes. The rush moves and nuances will come over time as he gets a little more NFL experience and he is already making an impact in games, so its not like he has been invisible from a big play perspective.
I won't go into team player/attitude/off-field issues because those are often overlooked when it comes to on field production. I think Merriman will be a good player, but I don't think that he is better than Ware or will be better than Ware... he may show up on Sportscenter more times, but I much rather have the complete player that Ware is working towards becoming, on my defense.