Without a doubt, movies based on games are FAR worse than games based on movies. There actually are some decent to good games based off of movies, though the average movie game is pretty terrible. As for movies based on games...My mind has YET to recall one that was actually any good.
There are a number of reasons why. Firstly, most movies based on games are lower budget money-makers that Hollywood decides to make every now and then when they want to make a decent profit with little effort. They are banking on the fact that fans will come see it no matter how batshit awful it is. They get no-talent directors to make them, give them a rather pitiful budget, hire some two-bit actors that no one has ever heard of, and usually try to make up a bunch of random changes to make them less true to the source material. Basically, Hollywood doesn't take game movies SERIOUSLY most of the time, and they are typically designed solely to make a quick buck off a pre-established franchise with a lot of fans, not make a good movie.
Secondly...Most games just aren't fit for any other media anyway because...Well...The gameplay is the only thing that makes them fun to play as video games. I'll be painfully blunt: Most video games are poorly written. Most video games have TERRIBLE, cliche-ridden plots with terribly developed, poorly written characters and fucking RETARDED dialog. There is a reason why "video game plot" is a major insult used by novel and movie critics. Even games that are praised for having very strong plots aren't all that amazing when compared to similar movies or novels. There certainly are exceptions, but the overwhelmingly vast majority of games have bad plots.
Case in point, Resident Evil. Honestly, who here plays those because of the characters and plot? When you transition a game to a movie, you lose all that interactivity, meaning that the movie must ride on its plot and characters...And frankly, most games fall flat on their face in that regard. So, all you have to look forward to is the action really...And because most game movies are pretty low budget, the action scenes just aren't as good as many other higher budget action movies. Really, you are better off just sticking to the game, or going to see a better movie with similar themes.
My point being, when you take the interactivity out of a video game, most of the time you aren't left with anything particularly worth spending two hours of your life watching.
I'd also like to mention that novels based on games are equally awful for the same reasons.