People often like to promote this one as somehow being more legitimate because it is awarded by the actors collectively.
Unfortunately, that's exactly the problem with the SAG Awards. As a rule of thumb, the larger the voting pool is, the more likely something other than the intended criteria will be a deciding factor.
In the case of the Screen Actors' Guild Awards, the awards tend to wind up going, not to people in cinema who are good at their jobs, but to the people that actors and actresses enjoy working with. When someone wins the SAG Award, it doesn't mean they're the best; it just means they're a really nice guy.
There are plenty of examples that could be used as an example, though perhaps none more illustrative than the following:
Fey has won four times for 30 Rock, in 2007, 2008, 2009, and 2012. And while she is certainly a very funny and very likable person, you wouldn't think so if your first exposure to Fey was through 30 Rock.
30 Rock is, to put it mildly, nothing more than a series of bad fanfics of Studio 60 on the Sunset Strip, full of repetitive, ham-fisted humor delivered by completely one-dimensional characters.
It's amazing that the show survived for as long as it has, especially since the first season often came across as nothing more than Fey ranting that her brilliance had not yet been recognized because she was surrounded by idiots.
Then again, maybe that's how everyone else in Hollywood feels too. And it's a perfectly legitimate feeling, especially given Hollywood's tendency to throw original scripts in the trash in favor of making movies like Movie 43, Battleship and The Oogieloves.
But even if that's the way you feel, it's generally better not to say so, much less to reward the people who say it first.