A lot of people will tell you that they aren't something you absolutely need, but I personally like having one. I feel that every little bit helps, especially when I clean out the cup from my skimmer and see how much crap it has collected.
That being said, I only have a 20 gallon, and don't really have a recommendation as to which one you should use. Mine is much too small for a 40 gallon, since it's rated up to 30 gallons. If you do get one, I find it's also best to get one that's rated for a higher gallon tank than what you have. If it were me, I would be looking for something that's rated for at least a 60 gallon tank.