I'd say go for the bivvying! It is a fantastic experience and makes a trip feel a lot more like an adventure! Only you know where your comfort zone is and how far you are willing to push it! Since trying the bivvy life, I have pretty much entirely given up on tents except for rock festivals etc. What's the point in camping in beautiful scenery and locking yourself away in a tent?! There is nothing more beautiful than lying in a bivvy on a cold winter night in the snow, staring that the mily way as you drift off to sleep. If you have a warm enough sleeping bag and mat, you can do so in perfect comfort!
Couple of tips:
- get a Rab siltarp as someone mentionned already for the rainy days, weights nothing, adds a lot of comfort
- if you don't plan on taking a tarp, pracitce at home. Play 'the floor is lava' and see how quickly you can undress, stash all your clothes in your pack without letting anythign touch the ground and get inside that bag. Because anythign hitting the ground will get soaked and the longer it takes you to get inside and zip up, the more miserable your night will be
(oh and get a large enough drysack to stash your backpack and all extra kit in during the night)
- put the sleeping mat on the inside of the bivvy
- take a bottle of water with you inside the bivvy or it will freeze
- if you use it during midgy season, zip it up as much as you can, leaving just a gap big enough to breathe through, that should be enough to keep the midgies out
- snow goggles combined with balaclava work well for sleeping in snow, or just slide further down into your bag and just keep a small opening to breathe through, but that way you miss the nice views