I'm going to add my view sorry...i'm strictly an outside person, i think they should have access to fresh air, grass and feel the soft natural earth on their little feet as much as possible and i just can't see how that can be done inside (personal opinion). SO my set up is i have sheds, insulated top to bottom with the silver insulated stuff, lino on the floor that comes up about a foot at the sides just in case of drafts, then wooden boards in front of that to stop any knawing at the lino (although they can't reach in shed 2 so no need for the boards, shavings and Hay/straw, there is a hutch then that they can get in and out of with loads of straw in the sleep area, and many hides, some plastic some cardboard all stuffed with soft straw, on the very cold nights i wrap these with fleece blankets too....temperature is tested every night at veg time and although i might be cold those little hides are toasty warm....there's other snuggly hay areas too so if they get too warm in there they can go somewhere else and still be warm but not too warm........
Each shed has a winter door (solid) and a good weather door (mesh) that can be inserted into the door frame, this means they can have fresh air even when it's not suitable to go on the grass, each shed has a "tunnel" into the field with a run at the end of it and then i have drainage pipe that connects to a different run further into the field, which i can move around to ensure always fresh grass....from how you describe your set up you could do something similar?
If they were indoors in the winter it would be much harder to adapt them in the summer, and they might miss some of the good days and grass......
Each to their own obviously but part of the piggy fun for me is building stuff to make sure they safe, warm and protected (i have power tools and a tool box that makes my mother very proud ha ha) and you'd be amazed how much you can do with pallets!
Katy