If you see the collection cup after the skimmer has done it's job, then you would definitly want to have one.
Without a skimmer, you should do more water changes but even at tha,t the proteins that a skimmer collects- floats, so even water changes would have a minimal effect on the actual proteins.
A skimmer is one of those things that has a lot of upsides and no down. So yes, a skimmer should be part of your set up.
As far as which one...
Do you want it to hang off of your tank or do you have a sump?
How big is your tank
and how much can you afford?