Here's my thoughts on both of them.
Slaking - "Could give Godzilla a run for its money when it comes to destroying downtown Tokyo. It launches a nice smashing attack on the Tokyo Tower but will loaf around whilst Godzilla takes down the Rainbow Ridge." -Serebii.net / Basically Hit Hard, but those who actually survive dies.
Snorlax - Same deal as Slaking, except that it destroys little streets instead of Towers, and actually consistently goes destroying (despite with a rubbish speed) unlike Slaking who likes to take a rest every while.
Both are HP tanks, Slaking with Physical Defense / Snorlax with Special Defense, Slaking with a legendary Attack and Snorlax with a respectable Attack. Rubbish ability for a rubbish speed. Both cancel out each other except the last part, which is unfortunate for Slaking.
But if Slaking had ANY fighting-type move of sort, it would win.