some, are definitely better than others, but just like i mentioned in your other post, what do you intend to do wiht the tank(what fish, or coralsand inverts), also what type of skimmer are you looking for, a hang on or an in sump? and what size tank, there are many skimmers ont he market, and although most do have their avantages for paying the extra money, some times you can't seem to be able to jsutify the price for your system(ex. almost no one really wants to spend 300$+ for a skimer for a 20 gallin tank, and so on)
and to add there, how much (if any) do you intend to set aside for a skimmer, you really should not skimp on skimming, but we can't always afford the actual best
HTH