I don't think anyone will ever win the down/synthetic debate.

Use what you're comfortable using.
Like Lawnboy, I use trash bags as well for stashing gear/food once in camp. I use a game bag inside a trash bag, and then hang in a tree. I like the trash compactor bags, or the heavy duty contractor bags, they hold up a lot better than a standard Hefty. The compactor bags fit like a glove in your pack for meat, but aren't big enough to put your pack in... The tricky part about trash bags is sealing the open end so they don't leak. I seem to always end up with water in my trash bags if its wet.
Sea to Summit makes some great light weight dry bags. I use 3 typically, one for cloths, one for my sleeping bag/pillow, and one for electronic gadgets.
Another critical piece of gear is a pack cover, it keeps your bag dry while hiking or in camp. It doubles as a meat sack if you forget your trash bags

, and it works to keep my butt dry when glassing or taking a break.
I have the Pinnacle, which is about 4-5oz heavier than the Helium. I love it, with a Sea to Summit reactor liner; I'm plenty warm to zero or less. Speaking of liners, I started using one about 4 years ago, won't go without one again. Keeps you bag clean(er), I can toss it in the washer when I get home.
I have a couple Mountain Hardwar Synthetic bags, they're just not as comfortable, and the one rated to 0 I'm cold at 20... They're also about 30% bulkier, but weight isn't a lot different. Synthetics have come a long ways in recent years.