I personally like the bivvy style. If I'm in a survival situation, that bivvy is going to keep me warmer than a blanket made from the same material; the bivvy goes all the way around my body, and wind can't blow in from the sides and rob me of heat. It's the same issue as a down quilt vs. a down sleeping bag. Generally a sleeping bag is warmer.

The thermo lite 2 bivvies are a tad bulky if all you're going to carry is a hip pack. Since they're so light, you could tie or otherwise secure the bivvy to the bottom (or top) of your hip pack, leaving the pack to carry items not so easily suspended. I like that they are multi-use, quieter, and sturdier than things like garbage bags or items made from space blanket material.

Re the bulk issue, another option is to just carry a day pack. I personally use a day pack since a hip pack was just too small for me to carry sufficient gear to provide what I felt was an adequate level of preparedness.

OTHER COMMENTS:
Basic foil emergency space blanket - good for signalling, can be used for emergency rain gear, not as good as bivvy. Noisy, single use only, prone to tearing. See Doug Ritter's comments: http://www.equipped.org/shelter.htm#Space
Tacoma Mountain Rescue Storm Shelter - Doug Ritter says he carries one. See http://www.equipped.org/onyrown.htm#ishelter
Extreme Pro-Tech Bag - Looks like single use, easily torn, and noisy.
AMK Heatsheet Blanket - Haven't used. Highly recommended in Cody Lundin's book, 98.6 Degrees -- How to Keep Your Ass Alive
AMK Heatsheet Emergency Bivvy - Looks like it is a Heatsheet bivvy.
AMK Thermo-Lite 2 Bivvy - multi use, low noise, less tear prone. In warm weather, I've used this for back packing along with a sleeping bag liner. Worked great.

ONE LAST COMMENT:
I still carry my old USGI coated nylon poncho. Of course it serves as a poncho, but it's big enough that it can protect your fanny pack or day back as well. It also makes a good ground cloth if you want to protect your bivvy from getting torn up. It can be snapped together length-wise to make a tube into which the bivvy can be inserted which will add more insulation and also protect the bivvy. A poncho can also be rigged as a lean to. Place your bivvy under the lean to and build a fire near your bivvy. The poncho lean to traps heat in the area of your bivvy. The lean to also can provide some shelter for your gear while you're in your bivvy. The USGI ponchos are a bit heavier than the sil or vinyl versions, but they're pretty tough and are multi-use. If you carry trekking poles and a little cord, USGI ponchoes can also be made into a little pup tent.