As a drummer for the past 12years, I have to say, for technical ability, Nicko wins hands down. In terms of being a "better drummer" - he simply is.
BUT! Lars is my "preferred" drummer. Back in the day, Lars (and also Matt Sorum, at that time, of Guns N Roses) was what got me into becoming a drummer, and who I based my style and playing upon. To this day I still use character traits you see in Lars playing as I have adopted them, and modified them into my own.
Being the drummer of my favourite band of all time, Metallica, Lars is still my favourite drummer, but I am not blinkered so much as to know he ins't the best drummer by a long shot. I know this, he knows this, as do many many other people - Lars will be the first to tell you he isn't remotely great and NEVER expects to see himself in Modern Drummers Hall of Fame...
That aside, I do adore his playing, and what captivated me is his raw determination, his energy, the level of dedication he puts into his playing.
Nicko tho, is a superb drummer. One of the few I would liken to a modern day John Bonham. He shied away from double kicks and double pedals when everyone around him was employing them, instead developing his own technique. One that, even with just one foot, embarrasess many younger drummers who use twin kicks/pedals (as I do myself). I have had the distinct pleasure of meeting Nicko twice now, and both times he is the utter gentleman - even with queues of people to meet, he still has so much time for everyone, and never "ushers" you away till YOU leave HIM, not vice versa. 2nd time I met him, he was in one of the artist bars at a drummers convention type thing, and I managed to wangle my way in. Plopped down beside him and we preceeded to have a good old fashioned chat for a good half hour, before I had to leave to see another drummer play. True gent, and all round "top geezer"
Not managed to meet my idol Lars yet tho - maybe one day