FYI, mainly for Bikenstein on using T3S (on future possibles) and billh on size.....
This is the istick 30W compared to the heatvape defender (rounded on both sides, btw), both with Kanger eMow megas (19mm) and a photoshopped image of a "2nd" defender with the knurled adapter reversed to accommodate the T3S, with the knurled part of the T3S 'hidden' in the "well". The height of the defender over the istick is mainly that adapter, which I really like, since I too vape (and have a cache of) T3Ss

The defender steps down to 5 watts (3.1 volts with a 1.9 ohm coil). I vape my lighter flavors with the T3S around 6.5-8W and the stronger DV Habana reserve at @11.5W. The istick 30W with a light flavor set at 6.5 watts.
The Defender is a tad wider than the istick, which suits my hand a bit better, esp. for the button firing. Also I haven't had the 'increased wattage' (mean vs RMS) with either the defender or the istick 30 that I had with the istick 20 which is now 'dead' due to the no contact/threading issue - or whatever it's called. No adapter fixes it - pretty sure I'd have to exchange the threading internally to fix.
The Defender has become my go-to mod, even though I have the VS rDNA-40. I took the defender on vacation and it rarely left my hand lol. And it is what I grab when I go out... along with a few eGos. That said, I have five main flavors I vape so the istick and the VS (JG Blend - Ahl. at 15-20W set at 390°) also gets much use. Very pleased with the istick 30 after the not so pleasing experience with the 20. My wife's 20 has never had a problem though, but she never unscrews the bottom of the emow mega she has on it, removing only the 'top' to fill.