Nytron...welcome to the forum! I have all three of these and here is what I think...
You know about the E. If you want all the flexibility it offers, then stick with it as these other vapes aren't built for that. I recently loaned out my E and gave a tutoring session on it and it reminded me of what a great vape it is since I hadn't used it in well over a year or two. But for me, the E would be third on that list since I never use the bags.
HI vs Solo brings a lot of things into play. Obviously, if portability is needed than the debate is over. The Solo doesn't even work when attached to the wall (unless you get a special plug), so you know it works well when walking about.
Efficiency wise, with just a little bit of use of both of them on my part, I think they are about the same so I wouldn't make that a differentiating factor for consideration.
Aesthetically, nothing beats the HI...my unit is the best looking vape I have seen (maple, right Alan? I forget what type of wood it is, only that it is beautiful).
For ease of use I would favor the Solo. You just push buttons and go for it. The HI is more sensitive to things like draw speed and depth of placement of the bowl in relationship to the heat port. This is a double-edge sword that somewhat defines the vaporist. Do you like it simple and constrained or complex but flexible? The HI is a little more complex to use, but then you get to tweak variables that allow for greater temperature range, etc.
So, to sum up, these vapes are different enough that I think you should think it through like you are doing. If you want something that is beautiful and has some flexibility in temps and the like, get the HI. If you want something brainless and portable, get the Solo. Another consideration is availability. The Solo you could have in your hands in a few day. I'm not sure how backed up Alan is, but there is a wait. Finally, if you collect vapes, get them both! Get the Solo now and get on Alan's list now for a later delivery. I think you will love either one, as both of them are among my three favorite vapes.