Both are great games but I think sonny has a little more to it than LS2, I mean Ls2 is great but you can only shoot zombies behind a barrier for so long before it gets boring, Sonny has 4 differant classes with abilities so that every character you make can be differant. plus like some people have already said sonny has been out much longer.
i like Sonny better for a # of reasons sonny has a story line, very good voice acting,the sequal should be even greater sonny has an unbelivably hard boss and secret level but LS2 does have a variety of weapons and is a definite improvement over LS1 short but does have a certain appeal to draw many back to it and there are a bunch of zombie shooters out there props to sonny for being a RPG zombie game
the last stand will come out best. it is more fun and has really good parts to it. not saying that sonny doesnt but they are pretty much the same game but TLS is just a better overall game.
well its hard to say, but i would go with LS2 because it is a popular sequel to a popular game, so lots of people are probably going to give it a good rating just because of that.