For instance, GAnalytic says 35% of my traffic use 1680x1050.
Large monitors make screen fonts look smaller. You surely don’t want to use 10-12px for a 1680x1050 resolution.
That’s why EM is the preferred method.
For some, certainly not for all.
Both methods have pro and con, depending on a variety of factors, including screen resolution capability, fonts used, browsers, platforms. There are just too many variables. That’s why there are so few hard and fast rules to web page layout and design. On recent sites I’ve been sticking with pixels for all elements (they’re usually large enough that they don’t really need to be increased in size) except entry text. Even then it’s a toss up. Some with pixels, some with em, though I’ve also been adding a text resize option.