"In line" skimmer vs "In sump" Skimmer

I am limited by space and thus an "In line" skimmer seems to make more
sense. But I am also limited in sump volume (3-5 gal max) and am concerned
about how difficult it will be to get an "in line" skimmer to work (keeping
the water level correct in the sump & skimmer). My tank is only a 20 gal,
which I have had good success with in the past, but I want to put a higher
performance skimmer on it than I have had in the past, thus the need for a
sump. Pictures are worth a thousand words, if you can send me to some, it
would be appreciated.

Thanks
Steve