Gunslinger gives you a mid-screen 3 way guessing game in the form of Stand Off Stance; however 2/3 of the follow up options are readily available (and imo done better) in Marksman as they both do more raw damage and have greater application - yeah Gunslinger lets you combo 11strings into SOS low shots (probably the only useful Gunslinger only combo there is) but it's not like EB is naturally lacking in low hitting combos...
This being said, Gunslinger does give you access to money shot, which helps create a synergy with caltrop set ups and allows you to pin down opponents mid screen - at least for a while. However as useful as money shot is, the relatively low raw damage and the fact that it doesn't really combo hinder its overall application in Erron Blacks' game - in all cases where I would use money shot, I could have used SOS 3/4 and/or either one of the Marksman rifle shots to better fulfill that role. And if it's caltrop synergy you want, mid-screen or up-close, Outlaw does it better.
One thing EB naturally lacks are full screen options - whilst Gunslinger has SOS 4 which causes EB to shoot and retreat to full screen, once he is there he can't really do much other than SOS 4 again. This isn't the case in Marksman; EB he has a retreating gun shot special AND can still threaten the opponent from full screen with his low shot rifle (or retreating shot again if you want) - and both of these specials do more raw damage to boot than their respective Gunslinger counterparts. So if it's projectile zoning you want, Marksman is the one to go for.
The saving grace Gunslinger has is SOS 2 (AA) follow up - it's got a sick angle, hit box and does 10% raw damage. However, in Marksman he gains an AA launcher that fulfills basically the same role AND lets him juggle afterwards. And in Outlaw, EB's neutral jump punch gets an extension, which ends up covering roughly the same angle as SOS 2...
Since all of my current bnbs can be done exactly the same in Marksman, I think I may just go and jump to that variation.