After sheep hunting all these years, and going thru different pairs of leathers, I've decided to go plastic and give it a try, what plastic boot would you recommend: Scarpa Invernos or the Lowa Civettas?, these are the two I feel would be the most comfortable and have the best flexability.

I have only used the Koflach. And lace them differently for different terain. In the really bad stuff what I like about them is there lack of flex. Esp. sidhilling. They give you a lot of confidence. Easier terrain and more distance
I will wear my leather style boots.

Can't speak for the Lowa boots, but have used the Scarpa Invernos extensively, not so much for hunting, but more mountaineering.
These are one of the few Plastic Boots that I can wear without turning my feet to pulp.
I've been through several pairs of Koflachs, and always the same...blisters...cramps...pain.
Have found the Invernos very good, just wouldn't want to do a 10 mile hike over flat ground in them, or any other plastic boot for that matter, as they are very rigid and designed for edging on steep slopes, or front pointing with crampons.
For big "Walk-ins" I have carried two pair of boots i.e. light/mid-weight hikers and Plastic mountaineering boots. Especially where there is river crossing work on the way to the hills.
Most plastic boots (Inner Boots) are a bitch to dry out once they are wet.
I think (But don't quote) the inner boots are replaceable on the invernos.
For my mountain hunts I now use Meindl Makalu Boots with high rubber rands and a double gaiter system. These work well in snow and ice, and keep you dry. Not too much of the leather comes in contact with the terrain/snow. (I also keep the snowseal up to them.)
With the double gaiters I use a short neoprene ankle gaiter, which seals out all the crap. (Shale, sticks and water to a point.) Over these I wear knee length snow gaiters. Without the inner (Short gaiters) I still seem to end up with all sorts of crap through my socks.
Anyway this sytem works for me, and is a good compromise between comfort, warmth, functional climbing and keeping dry/breathable.
My 10 cents worth...

I use the Invernos because they were the widest plastic boots I tried on. They fit very well and in a way are TOO good because with them on I can easily climb up something that I can't climb down. Serious here-they are mucho bigo better than any leathers I have worn.

I trained for a sheep hunt while living in NC and was forced to do most of my training on the blacktop. I trained with a good bit of weight in a pack while walking on basically flat blactop. I would have to dig out the boots for me see exactly how I laced them, but left the top one or two or something of the top laces untied. If I had them laced all the tight I could still walk on flat blacktop, but it was not the most fun walking I have ever done. After trying them in the field I cannot imagine a sheep, goat, or similar terrain hunt without my Invernos strapp on.