The AMK Thermo Lite bivvy is a little bulkier than a heat sheets bivvy but it seems a lot more durable. I also like that I can fold it in thirds and roll it up again after use. I've never been able to fold up properly anything like a space blanket after use. There's also a little vent at the foot which may help with condensation.

For what it's worth, here's my gear list from a backpack I took last month:
Gear List Comanche Backpack 6/11/2010
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-BACKPACK
Osprey Talon 33
Rain Cover -- Sea to Summit

-SHELTER
Big Agnes Seedhouse SL 1 (fast pack)
WM sleeping bag -- 32F <== Attic
Neoair pad
Inflatable Pillow
Ground cloth <== Attic?

-COOKING & EATING
Snow Peak Ti Sierra Cup
MSR Titan kettle
Cat can alcohol stove or Optimus Crux gas stove or (heavy!) Trangia 27
Alcohol in squeeze bottle or 4oz tank of gas
Aluminum cutlery set
Plastic bowl

-FOOD & WATER
3.0L Platypus Big Zip
1.0L Platypus water carrier
ClO2 tabs
Yellow Ursack
...See Food Plan...

-CLOTHING
Maui sun hat
Army Poncho
Patagonia down anorak
l/s Nylon shirt
Nylon hiking pants
Wicking T shirt
Zip T neck
Long john bottoms
Salomon Trail Runners
Fox River socks -- two pairs
Heavier socks for sleeping
Light weight fleece cap
Light weight gloves

TEN+ ESSENTIALS
Harrison Angeles High Country topo map
Neck Compass
Head Lamp <== Charge your batteries
Spare AAA batteries
Sun screen - 1 oz
Sun glasses
Chapstick
Lighter
Matches
Coghlan's Tinder
Leatherman Blaze or SAK & CRKT Lake
FAK+
Emergency Kit
PLB
Duct Tape
Large black trash can liners
Whistle
Signal mirror
Suunto altimeter watch
Head net
Mosquito repellant
Knee straps

PERSONAL MEDICINES
Glucosamine Chondroitin
Vitamin C
***Meloxicum*** <== DO NOT FORGET!!!

-MISCELLANEOUS
Toothbrush
Toothpaste
Deodorant
Baby wipes
TP
Hand sanitizer
Toilet kit (holds TP and hand sanitizer)
Snow stake (trowel)
Misc kit (holds matches, tinder, and duct tape)
Large Zip Loc trash bag (1 gallon)
Spare bandana
Car & house keys
Cell phone
Wallet
Pen
Walkie Talkie <== Charge your batteries
Komperdell trekking poles
Panasonic DMC-TZ3 (Lumix) camera
Spare batteries
_______________________________________________

THE TEN ESSENTIALS
M Map
C Compass
L Light
S Sun protection
E Extra food & water
E Extra clothing
M Matches
F Fire starter
K Knife
F FAK