SNL has stars and bad writing but MadTV has less stars and good writing.
A MadTV sketch, develops and changes with jokes WRITTEN in. SNL sketches all have the same formula to them. Think of a funny idea/joke. Play out the funny idea/joke for 2 and a half minutes then make a slight tweak to the joke for the last 30 seconds.
There are, of course exceptions to SNL sketches but I assure you the majority will follow that formula.... but don't get me wrong, the formula does work. Best of Will Ferrel and Chris Farley are hilarious. It's just a question as to whether or not the joke is funny enough or the character is interesting enough to make last 3 minutes. I always liked Weekend Update because it would have written jokes in it... nowadays its probably the only thing worth watching on the show.
But yeah I enjoy MadTV more... Even now.