It comes down to personal preference, of course, so only you can answer this for yourself. Most UL backpackers that use bivys with tarps do so for extra rain and bug protection when needed. If you are OK with the confined quarters of a bivy go for it. Others prefer more space and use the various tarptents mentioned, which come with a little weight penalty. I'm in that category so I added netting to my tarp essentially making it a floorless tarptent. I also made what I call a storm door so I can cover up most of the large opening if needed (my tarp doesn't have a beak).