Each also has a 'black' cover that matches the fabric of the shack. So, if you'd prefer less light or only need certain windows open to see, you can leave the fabric over the plastic. This is how I fish most times, only opening a few windows for light (and I don't tip-up fish - if I did I'd probably strategically open a few that face my tip-ups). This aides even further in helping keep warmth in.
I upgraded to the Fatfish 6120I last winter after a ton of research. I went with the 6120 for the much bigger size (about 20 square foot more) without much of a price increase, compared to my previous hub that was a smaller "square" style. We fish with 4 people when we use the hub, and this size was just a tad small. For 2 - 3 guys it was awesome. The 6120 is perfect for 4 and it would probably even be comfortable with 5 or 6 guys because of the shape (6 sided vs. 4). Not sure how big you want, but I would very highly recommend the 6120I after a season of heavy use last year.
When looking, I couldn't find anyone else with a shape like the 6120I (maybe I missed one or someone has one now?) so my decision was very easy. The YKK zippers were also a must after struggling with the zippers on the old one. It just didn't zip great when the shelter was up and fabric pulled tight. The YKK's are the best out there, IMO (not just for shelters either).
I also wanted the black interior that Eskimo uses. We set the hub up over cribs we fish in 8-10 foot of water for crappies. The dark blocks the sunlight very well and helps the crappies act less spooked - another tip is to actually fill your hole with ice chips to block sunlight when in shallow water - but this is a whole other discussion =) - so I preferred this. When the windows are opened, it has more than enough light if we want to fish that way. We run a single LED lantern on the roof when it gets dark and it works great. Again, this is just a preference thing though and may not at all fit your needs! I know a lot of guys who actually 'black-out' the inside of their hub shelters, so this is a welcome feature.
Your Question on Windows: I currently have my 6120i set-up in my basement to re-stretch for the coming winter after being stored for a while. So, I ran down and took a picture looking out (sorry it's a bit dark, the unfinished basement is my fishing storage area, so not a ton of lighting). But, I'm 5'8 and I can see perfectly out the windows when sitting, as you can hopefully see in the picture.