Thanks. I have a synthetic, snugpak tactical 3, and I get cold in it at +2c, despite it being rated at -7c comfort level. (tactical 3 is 1.7kg, packed 23cm x 20cm). So I was looking at down because I don't want to increase size or weight, preferably reduce.
I bought a Snugpak Osprey 12 back in the nineties, and it was never as warm as advertised - and a look around the internet shows that Snugpaks are notorious for their temp ratings. And certainly no longer cutting edge.
I let my son use it last year for a school camp. Its lost a fair amount of loft (to be fair its decades old), but it still took him and a teacher to get the bag back in the bag, and they did not entirely succeed - thats the problem with synthetic bags - they tend to be relatively bulky. My ME down bag is way easier to pack
As for the three, the OEX has a lower fill , so yeah, its going to be less efficient than the other two, and that temp rating looks a bit suspect. There is a really good overview about fill power and loads of other stuff about down on the
PHD website (OK, so thats looking at Savile Row, but we can all dream). But fill power and percentage of down does make a difference, and that costs.
As for the other two, as others have pointed out, the bags, are the same weight of fill, with the same fill power, so are probably pretty similar. But its a bit like having the choice of two suits, pair of shoes or TV's - it comes down to
fit.
I remember a comment a couple of years back that thought the Alpkit was great value, but perhaps a little on the narrow side. Rabs are easier, because pretty much every Go Outdoors and most other decent outdoor shops should have at least one of their bags, most likely the 900. OK, thats going to be out of bounds for the next couple of months, but your not buying a pig in a poke. I am a big Alpkit fan, but like clothing, bags are something that I like to try on. I tried on a Mountain Hardware Lamina in Cotswolds a couple of years ago - great bag but personally, I found the foot tight. Its very individual.
Both have a good reputation, but big brand down manufacturers tend to be a bit cautious with regard to the 'offical' test figure, and will often have a 'good nights sleep' figure which might be a bit lower, because of their experience, etc. So I would probably chose the Rab, but there might be a really good Alpkit deal. If your close enough, you can go to one of their stores our their HQ outlet. At least at some point....
Roughly £199 to £219 (assuming you go for the 15% discount at Go Outdoors) is good value for a roughtly minus 12-13 bag which comes in about 1.5kg. But if your willing to spend a little bit more (and looking at retailing at the moment, I suspect deals will be ramping up as stores try to tempt us in order to move stock), I would throw another one into the mix - a
Mountain Equipment Classic 750.
This bag comes in at 1.33kg, with a fill weight of 718g. But the fill power is higher, at 700, at a minimum. My ME Lifeline jacket's swing tag said that its fill power was actually 725, although its officially 700. And while the Rab is using 80/20 fill, they are using 90/10. So its a bit lighter and a bit less bulky, but its official comfort level is minus 12 (and ME reckons a good night sleep at minus 14), pretty much the same as the other two.
Trekkit have it at £225 at the moment with free delivery. The ME Classic range (I have the 500) is essentially their equivalent of the Rab Asecent range - entry down bags, but solidly decent with a good brand name.
Good luck - bags are one of those subjects everyone has an opinion on, and until you have bought it and used it, your not sure if its does what you think it will. Which is potentially a lot of money that might go to waste. But the RAB, Alpkit and ME are all solid bags