They can both cause weight gain. Theoretically, Lexapro has fewer side effects, but it probably doesn't. Depends on the person. What they did was cut Celexa in half to create Lexapro, and thus were able to get a new patent. Celexa is a lot cheaper, therefore. If Celexa is working, I probably wouldn't switch; if it isn't, then I probably would.
Just an FYI---Lexapro contains the same ingredient as Celexa. Hence the "lex" in the name. Some people have better luck with one med vs another, a lot of times it is trial and error. I have had great success with Lexapro, but everyone is different. There are some initial side effects, but once they subsided for me, it really helped.