I'd go for the Integral Designs bivy, simply because I've used the ID Salathe and liked it very much. In fact, if you're planning to use it in summer conditions, you might want to consider the Salathe. It's only $15 more (but a bit heavier), and it has a panel that zips open on both sides all the way down to your waist - a real godsend on a hot, steamy night.

If you're not concerned about being able to close it completely up (you said you were using it with a tarp or in a shelter), you might want to look at the REI Minimalist - 1 pound and $100, when I bought mine. It doesn't have the chest zip, but it does zip open like a sleeping bag. It was my first bivy, and it worked quite well - got a bit warmish in August, though.