I find both sites useful in their own way but every once in a while I am astounded at a rating at RT considering that so many critics contribute to it.
Let me give you a few examples of times that I think the critics on RT got it horrifically wrong.
Watch It rating = $2
I liked Spider Man 1 & 2, in fact #2 was a great movie, but Spider Man 3 was terrible. The writing was on a much lower level than the previous two superior installments and the dark side of Spider Man was extremely unconvincing. This movie contained some of the worst scenes I've sat through in years and yet somehow 62% of RT critics recommended it! Are you kidding me?
Watch It rating = $4
Live Freed or Die Hard was OK. The complete and total implausibility of most of the action scenes lost me though and the villain was lame. An 80% recommendation from RT is shameful.
Watch It rating = $3
I've taken a lot of heat for my low rating of Knocked Up. I still say that it's a simple and vulgar movie with far less laughs than I expected. A 91% level on RT should be reserved for Oscar contenders not comedies that revolve around masturbation and poo jokes.
Watch It rating = $8
Transformers was a fun and entertaining movie. One of the knocks against it has been that it was weak on plot but did people really expect more from a movie based on a cartoon which was based on a toy? While 57% of RT critics saw it as good enough to recommend I don't know what the other 43% were hoping for, especially when 62% of them recommended the retched Spider Man 3.
Just my opinion though.