I too have both. I like them both. And each serves a specific purpose for Me. I have a 3 Speed Heat Cap (low, med, high) and for Me, the Heat Cap provides an intense sort of baking heat. Sometimes, I feel like I want that intense feel.
For Example 2 weeks ago when I self relaxed, I used the Heat Cap for my Protein Treatment. It Felt Wonderful. And When it came time for me to DC (with Moisture) I used the Steamer. And it Felt Great.
I love my Steamer. And For Me, it does something entirely different. Imagine a Tea Kettle or a Steam Iron. It is Emitting Mist of Steam, so it's a totally different experience. Totally Moisturizing.
A Baking Heat Or Steam. Oven Or Tea Kettle. An Iron on Steam. An Iron on Dry Press.
Both are quite different. So you see, I get Great Use out of both.
However, If I was looking to purchase Just One, it would probably be the Steamer because of the Moisturizing and My Hair needs alot of Moisture because I use alot of Protein.