As someone who has watched both guys during their stints in Townsville, Gleeson is the better coach.
Despite his outwardly laidback appearance, Dennis grates on veterans and then they tune him out so he needs to have a young team. He has very questionable rotations and lineups, and has trouble getting his teams up for big games. I also don't like the way he allocates shots and plays: he seems to force the wrong guys into shouldering the offensive load most nights (eg. trying to run plays through Mickell Gladness or Clint Steindl ad infinitum). His teams have been consistently terrible on the defensive end and not great on offense either.
Gleeson on the other hand is solid in most areas, but not particularly great.
Paul Woolpert is better than both of them, but that's another story.
Leaguewide everyone goes on about how Shawn Dennis is a smart coach but as someone who has watched him very closely I really fail to see what they're talking about. Poor defense, poor offense, poor rotations, blown leads, veteran players hate him, guys' roles are poorly defined so they play inconsistently, schemes not suiting the abilities of the playing roster. And I'm sitting here and scratching my head trying to think about what parts of the game he coaches well.
I feel he is trading somewhat on his reputation as Rob Beveridge's lead assistant during Perth's dominant run.
I'm not saying he's terrible, but he isn't this "great coach getting the most out of an under-matched team" that everyone outside of North Queensland makes him out to be. People watch so little of the Townsville games that their capacity for analysis of their performance is somewhat limited.
Case in point: up until Blanchfield moved to Melbourne just this season, "Mitch Creek or Todd Blanchfield?" was a legitimate debate to have for lots of NBL fans.