Get a firstlight, and a cheap bivy. You'll use the firstlight more often, and the bivy when you plan to.

Bivy sacks are really not that comfortable. Here is the bivy I went with....

http://www.prolitegear.com/rab_ultra_bivy.html

6.7 oz. Welded seams... does the trick when I need it. I mostly take it on summer stuff, when I am trying to keep the weight down.

I just bought an OR Alpine Bivy. I used my friend's on Rainier last year and wanted my own. It's completely waterproof, has a single pole to keep fabric off your head, and has mosquito netting. Pretty much the perfect bivy, in my opinion.

The Integral Designs Unishelter has worked well for me.