That's a question a lot of OMers are asking themselves and each other as temperatures fall, so what are the pros and cons of each and which works better for UK conditions.
Down has pretty much the best warmth to weight ratio ofany clothing insulation. It packs down into a small space, traps a lot of air relative to its mass and is also surprisingly durable if cared for properly.
All of which is great. Down does have an Achilles heel though, it doesn't like damp - when down gets wet, the surface tension of the water overcomes the ability of the tiny down fronds to expands - think wet hair in the rain - and the down loses pretty much all insulation value and collapses into a soggy mess.
Many down jackets have water-resistant or even waterproof outer shells, but these are designed more to cope with wet snow and the odd very light shower than proper rain which will, inevitably, get to the down in time. That's the case even with fully waterproof outers, eventually rain will get in through the neck for example, though they will cope with some wet weather use particularly if you keep the hood up. Ask yourself though, will you really need the warmth of down if it's warm enough to rain rather than snow?
Because of this, down works best in cold, dry conditions and inside when the temperature plummets. It needs more care than synthetics, a waterproof stuff sac is a good move, is more expensive and harder to wash.
Primaloft is the best known 'down-like' synthetic filling but other synthetics have broadly similar qualities. Synthetic fibres aren't as light or compressible as down and have an inferior warmth to weight ratio, but they do have two big pluses:
First they are often hydrophobic and retain far more of their insulating capacity when damp, so you can layer them over wet clothing without worry and not panic if it starts to rain. Second, they're easier to care for - you can simply chuck them in a washing machine, unlike down which needs special care.
For a lot of UK use, that makes synthetics more user friendly and more versatile, though they do lack some of the fluffy weightless warmth luxury that makes down so attractive.
As you've probably surmised, for a lot of UK active use, synthetic insulation is a lot more user friendly than down. On cold, dry days, down has unrivalled warmth to weight properties, but add UK-style rain and humidity to the equation and synthetics, with their wet-friendly properties and knockabout care qualities make more sense.
Logically then, you'd expect synthetics to outsell down in the UK. In reality that's not always the case, Rab, for example, sells far more down than synthetic jackets, probably because a lot of down-type jackets are used most for apres-mountain activities like walking to the pub or sitting around a bunk house when water resistance is a lot less important.
The Bottom Line
The bottom line? In our opinion, for versatile active use, go for a synthetically-filled insulated or belay- type jacket which you can layer over other kit even in damp conditions. For cold dry alpine or trekking use or for simply going down the pub at the end of the day or sitting around in huts, tents and bunk-houses, then down is lighter and warmer for its weight.
Be realistic about your needs and make you choice on that basis and you won't go far wrong.