I know space but what other benifits? Where would you place a sump skimmer in the sump (for a 90 gal with built in reefugium in sump)? I've seen some skimmers placed first in the sump and some at the end.
Outside skimmers can also be larger (like Rich says, Bigger is Better). Some people like to show-off their marine equipment. It may be easier to remove the cup and clean the skimmer.
In-sump skimmers should be placed before your refugium in the water flow.
not much diffrence i use a deltec in sump skimmer.. works great one benefit of a in sump skimmer is . if the skimmer ever over flows it does it in the sump and not all over the house.. like my deltec they use only one pump insted of two.. what kind of skimmer are you looking to get..
I think the only mechanical difference is, overflow protection, space savings and skimming where you want - some theories say that skimming after your fuge takes out the nutrients the fuge puts in. I personally don't know how that scientifically works but have always had my fuge after my skimmer and enjoyed it's success.