An RTA for a tootle puffer? The KFL+v2 or the Erlkonigin. A really nice Erlk clone can be had for a few dollars.
EK Style Rebuildable Tank Atomizer (8ml) White
For $5 + shipping, I highly recommend the Erlk clone. And you have the choice of 4 different colored tanks. Simply be patient in learning the build (took me 3-4 days). I greatly prefer my Erlk clone to my KFL+v2; better flavor, far more airflow options, practically no leaking, easier to build (after you get the hang of it), smaller, lighter, higher juice capacity (6ml vs 4.5ml), top-fill, and a cheap nano kit (makes for a tiny topper with a 3ml capacity). iBTanked has their great glass tanks for it starting at $5. It's my only RTA that looks great on my iStick 20W.
I use my KFL+v2 in the house only. It works great. But I've been unsuccessful in wicking it for the performance I want without leaking. It will leak when lying on its side. A deal-breaker for me and my out & about rigs.
I would not recommend a K4. When it works, it's great. Way over-engineered IMO. Far too fiddly and unnecessary for us tootle puffers. I sold my Tobeco K4. It just wasn't worth the hassle. I get equal or better vapes elsewhere.
Both the SubTank Mini w/RBA head and Eleaf Lemo 2 look very promising. The latest STMini has a 4th, smaller airhole perfect for MTL tootlers. The Lemo 2's airflow can be shut down completely. The rest is up to your build and choice of drip-tip. Both sit @ approximately $30. I'm waiting for massive sales to get both and test drive them.
Hope this helps.
(It's on the iStick 20W in the photo below (on the right). KFL+v2 on the iStick 30W.)