When shopping for camping sleeping bags one of the key feature differences is what kind of insulation is used inside the bag. The choice boils down to down or synthetic insulation. Both choices have pluses and minuses, and either can be a good value depending on your needs.
Down is the traditional insulation used in camping sleeping bags. One downside is that it stops insulating when wet. If you are in the middle of the woods and your bag is completely soaked, you will be very cold.
The alternative to down insulation is synthetic material. One cool property is that it will keep you something warm when it's wet. Which is something down just can not do.
However, my take on the wet issue is that it's mainly sales hype. It's hard to completely soak a sleeping bag though. I've spent a total of thirteen months backpacking, and I have never soaked camping sleeping bags or even come close. Even when I fell into a raging stream in the High Sierras, my 3 season sleeping bag was not soaked the whole way through. I even slept on snow that night and needed warmth!
Synthetic insulation is hypoallergenic. It's also generally cheaper than down. Plus if it does get wet, it dries quickly.
Comfort is king when it comes to sleeping. If you are not getting good sleep, you will be miserable the next day. Down drapes over the body better than synthetic insulation.
Synthetic insulation is heavier than down. Down gives you a lighter bag for the weight. This is why the light sleeping bag market is dominated by down bags. If you are trying to keep your pack as light as possible, down is probably the way to go.
One other issue with synthetic insulation is that it breaks down much faster than down. Your sleeping bag just will not last as long. If you're the type that likes to buy new equipment every few years, I would not worry too much about this. If you're in the long haul, the cheap synthetic insulation may not be such a good value after all.
In the end, either down or synthetic insulation in camping sleeping bags will keep you warm at night. If you already have a synthetic sleeping bag, there's no reason to rush out and buy a down one and vice-versa. If you are in the market for a new one, remember that synthetic insulation is trying to emulate and replace down. Either can be a good value, it depends on what meets your needs.