Trust me, there will be a large margin between Resident Evil and Left4Dead. While they may both be zombie oriented, Left4Dead boasts a number of unique features that you won't find in the RE series or Dead Rising for that matter. When it comes to horror gaming I do my research. For instance, when they call L4D a survival game, they mean it. Going it solo will get you killed, and fast. Tactics will play a big part in L4D, meaning that any masterchief strategies will also end up in you kicking the bucket. One of the features I find most interesting is the class system. Not much has been revealed on it, but a player will get to advance their character by choosing a certain path, gaining expertise in certain skills and whatnot.
To say L4D will be very similar to Resident Evil is for the most part incorrect. Sure they both have zombies and other bizzarre creatures, but when you get past that your looking at two drastically different games. I love both Dead Rising and Resident Evil, but something tells me that Left4Dead may just end up kicking their asses.