If you have the right number of sprinklers to use no more than the capacity of the 3/4 inch lines in each zone, running a loop of the 1" pipe really won't gain you anything. If the sprinkler capacity exceeds the carrying capacity of the 3/4 inch poly, then a loop lets you feed in from each end and cut the effective number of sprinklers on each line, however you could end up with a pressure drop going from a 1 1/4" main line to 2 1" lines to feed both ends of the 3/4" lines. Better to hear from someone with more than just minor experience in varying sized lines and volume/pressure relationships.

Both the white PVC and the black poly get stiffer/more brittle when cold than when warm, though I think the white PVC is worse. The main thing is to blow them out or get them laid on a gentle slope so that you can thoroughly drain the lines in the winter. Buried underground, brittleness really isn't an issue, and the black poly still retains some flexibility. White PVC is often used, but any ground shifting or lawnmowers running over sprinkler heads and pushing them down into wet soil can break the PVC. Limited experience has been that black poly is more resilient over time.

You also want to make sure that you get the correct poly pipe because you don't want it to pop due to pressure. You should be fine if you bury it bellow the freezing point.

Here is a link that might be useful: do it your self site

i use 1 in.main line and 1/2 in.runners the length of the beds or garden.then 1/4 in. tubing to the sprayers or drippers. as for lines freezing, i blow out the lines in early winter. no problems,i live in the buf. n.y. area. if you bury them to deep and have a leak you can imagine the problem you will have finding it.i get my tubing and fittings from berry hill irrigation.