mtnkid pretty well summed it up.
With that said I have several shelters for different kinds of trips. This weekend the TI Goat Bivy and tarp are getting the nod, last weekend it was the SL-3, the weekend before that was the BA Copper Spur 1. Scouting trips i used Gossamer Gears "The One" shelter or the Rainbow from Tarptent. To show you how close in weight they can be, The One shelter weighs 23 3/8 ounces with ground cloth and stakes, my Bivy and 5x8 tarp weigh 20 1/8 oz. with 12 stakes and necessary 2mm cordage. The bivy setup is going this weekend as I plan to be on the move a bit more and finding a place to pitch a tent is a bit more difficult in this area and I expect no mosquitoes and if there are, the bivy has a net. The One shelter is also very noisy to set up and take down due to the Spinnaker fabric. I will say this, Bivy's suck without a tarp in a downpour, been there done that! The best bivy I ever owned was an ID Unishelter, all eVent fabric and would breathe very well and was bombproof. The OR Goretex bivy I had seemed to have alot of condensation issues in certain conditions and was a pain. Good luck with your decision, there is something about a bivy that to me is just fun and I am pushing 50 yrs. old!