I use two units... The Sigeli 100w and the Cloupor mini. Both have the ability to go .5 ohms for the subtank, and both have replaceable batteries, which is a great option to just swap batteries and charge the older ones instead of having to wait for your unit to charge, or be tethered to a cord.
The Cloupor mini is as small as an Istick 20, has a replaceable battery, and can be charged via a usb cable. It's a good "on the go" device. The only downsides, are the battery compartment does seem to come off just a bit too easy, and a .8 second delay from power off to puffing, and the center positive pin is not spring loaded or reverse threaded. which means that if you over tighten the atomizer, you're pushing the pin deeper in.... Neither of these issues are major gripes, and I've been pleased with it.
The Sigeli 100W I have is a first edition. It does not have a spring loaded center pin, but it does have a reverse threaded center pin. I personally don't like the spring loaded pins, but it seems people had issues with not remembering, and ruining the center pin. It hasn't been an issue with me, and I'm still on the original pin 6 months later. Mine has the option to screw on the battery cover, but I never use the screws. Luckily, mine came with a panel sticker to hide the holes. The newer versions of the Sigelis seemed to have improved on my design. They elimitanted the screw holes and replaced with even more magnets.. which will not come off unintentionally. They use a spring loaded center pin, which seems to be better for most people. They also streamlined the box.
I did have an Istick 20, and I'm glad I switched to the cloupor. The Istick 20 is still a nice device, but to me, I wanted a replaceable battery. The Istick 20 I had was rock solid performer. The istick 50's have had issues with "constant firing", but I believe they have fixed it since then. I'd still be leary since there's no way to test to see if yours will constantly fire or not... so, it's risky in my opinion.