How soon are you looking to buy? You might know already that the HI has quite a waiting list, and you need to get on the list and wait a while until your turn comes up.
Nanos and UD's can be readily purchased. UDs, probably the fastest.
I've owned these 3, and have greatly enjoyed each one. They're all pretty, they're all efficient, and they're all handy to have around the house.
To the best of my knowledge, the Nano is AC powered and has an open core with an aluminum casing inside surrounding the heater to keep heat from reaching the wood too much, and it has a ceramic heater. The UD is DC powered and a closed core with insulation. The HI is DC powered and an open core that does not use/need insulation...
As far as I know, both the v2 Nanos and UDs use glue in their construction. This is completely isolated from the airpath and shouldn't be a problem, but I think it is something worth knowing. Someone please correct me if I'm wrong here.
The Nano can be more convenient, with its built-in temp control knob, but IME, you have more precision / control using a vvps to change your temperature with a HI / UD.
I've never had any trouble milking any of my glass or anyone else's glass with the Nano, HI, or UD. If you really like to milk a piece, the Nano might have the most power. And you'd want a SS heater cover HI, not the glass heater cover. Any UD will make milk.
All of them provide excellent customer service, so you can't really go wrong there, IMO. And all of them offer plenty of stem options, with the HI possibly offering the most...
I have never found to have much preference as far as their draw resistance goes. They all hit fine and smooth and dandy to me...
Based on what I can recall, the UD always hit me hardest...
The craftsmanship on all three are great. Lookwise, obviously, you're getting into matters of personal preference.
I currently have only a Heat Island, but can see myself with another log or two somewhere down the road...
Whatever you decide, I say you should contact Alan of Heat Island and put your name on the waiting list, as it's free to do so and you never know what you might want later on down the trail. Even if you're not ready when your time comes, you can keep your place until you are ready.
EDIT: More can be found here:
DOUBLE EDIT: You can buy wood for any of these logs from
@Snappo here on FC...