Here's my issue: I have two sleeping bags that generally cover my all-year use: a a left-zip 30(F) degree bag, size regular and a right zip -20(F) bag, size long. If it matters, the 30(F) is shelled with Pertex Quantum and the -20(F) is shelled with an older Gore Dry-Loft.
I would like to purchase a single bivy for use in all seasons and hence with both bagsI have never used a bivy before. Use would be under a tarp or other floorless shelter in all seasons. (And yes, I know Mike C. does not use a bivy on his NOLS trips – he's awesome like that). Full bug protection is a must.
I have been looking at the MLD Superlight and the TiGoat Ptarmigan. The MLD has a partial side zip, which I like; but it is not long enough to fit the winter bag without going a size up. The TiGoat does not have a side zip, but will fit the winter bag without a size increase. As stated, the lengths are different on the bags, as are the zips.
How much importance should I give to the side zip? Right now, it looks like I either need to go with the TiGoat or a large SuperLight.
Any ideas?

