I've been thinking about this recently. I think a few considerations are:
How are you going to carry it?
Is space at a premium?
Cost?
Temperature during use?
Temperature is the biggy, are you going mid summer and sleep warm anyway? In that case a 1-2 season synthetic can be bought cheap and they pack down quite small. If you're going for a 4 season plus bag to keep you warm in the middle of winter a synthetic would be huge and down may be you're only option. Then think about carrying it, if it's going in a large pannier bag, and will be on it's own, you could probably afford the space for a cheapo synthetic, if you're going ultralight then down again may be your only option. Another factor that I take in to consideration is cost. How much are you willing to spend?
I'll be adding a few bits to my blog later on this evening. I'll be posting up comparison pictures of a cheap crappy tesco synthetic microlite bag versus a cheap crappy Vangor wilderness 250 bag.
Both bags come in at the £20 mark, which is dirt cheap.
The Tesco bag has a comfort rating of 13*c, the Vango is 5*c.
The Vango weighs 1.3 kg, the Tescos bag is about half that. The Tescos bag also packs to about half as small. It'll make more sense once I've uploaded pics and weights etc.
I've done some research on liners and from what I can tell it seems they're primarily useful for preventing the need to clean the bag, and offer little to no insulation benefits. The Thermartex Bedding Blanket however, is supposed to work well. I think I'll be grabbing one to combine with the Tescos bag for spring / summer. During colder periods I'd just have to bite the bullet and lug the Vango bag around.