I think it all boils down to trapping warm air above you and your pad. For the weight, high-loft down does this the best. I favor an oversized bag that allows me to wear layers inside without compressing the loft.
Be careful of ratings. If you look in Campmor's catalogue for example, you will see bags using the same insulation for which the bag with less insulation weight is rated by that manuafacturer higher than the other bag. I trust the ratings of high-end down manufacturers like Western Mountaineering and Feathered Friends. These are probably out of your price range, but serve as a point of comparison. In the synthetic arena, I think Big Agnes ratings are pretty good.