I echo what the other agents have said...both are wonderful places to live. Laguna Niguel probably has more allure to it with its name than Aliso Viejo does. Laguna Niguel is more established, older and generally more expensive homes...you get into a lot of areas that have ocean views.
Aliso is a younger community both in the age of the homes and the overall age of those living here. As mentioned earlier, many homes have great views of Saddleback or of the Canyon area. I, personally, have loved living in Aliso Viejo for more than ten years now...I don't plan to ever leave!
Good luck! If you need any assistance with your home search, I'm happy to help.