I ran both S4's and S5's. The S4's do seem to tend to do better with heat. I think it partially has to do with it's design and full case. It gets hot air out decently. Also I had a fan pushing it to so it really worked good.
Those S5's though are pretty warm when you run them. The 1 fan with open design just allows heat to go everywhere. The did improve it on S5+ but does not give S5 users any hope.
But I would focus on the S5's like you are doing. Also depending on your location winter is coming so you will be able to get free cold air in certain areas which is pretty great.