Okay, they're both "infotainment".
I prefer Mythbusters over smash lab for the following reasons:
A) The MB folks are characters. Adam, Jamie, Tory, Grant, and Kari all have personalities. It would be fun to spend a day/work with them. The SL 'hosts' could be played by cardboard cutouts and no one would notice the difference.
B) MB is 'serious' fun. They 'know' that what they're doing is not totally scientific, so everything is done with kind of a wink and a nod and a lot of what they do is just for fun. SL is way too serious and most the time it seems like they have solutions in search of a problem.
C) MB is done out of part of a rented building and they occasionally disturb their neighbors. (I could just imagine looking out my office window and seeing the MB folks in parking lot and saying to myself " I wonder WTF they're up to this time !?!?!?!?") Okay, I get it that SL is a production of DOWNEY STUDIOS, and that DOWNEY STUDIOS IS REALLY FREAKIN' HUGE. (although there never seems to be any cars in the parking lot.)
D) I know someone will say that Kari is hot. Sorry, she doesn't trip my trigger. (Neither did Scotty). I do have to give SL the nod for having a better looking female host. But Kari would be more fun to work with/has a good sense of humor.