point to bear in mind, especially with down bags, if it's really cold don't be tempted to get your face inside the bag, the moisture in your breath goes through the inner shell, through the down but then condenses on the inside of the cold outer shell, ie, in the down, making for cold, damp, ineffective down
I'm 6'3 and use the Lowe alpine with the adjustable back,you get measured in the shop and the sack set up to suit.( although obviously you can fiddle with it when you get home, or if you lend it to someone of a different size.) the sack I've got is 75 + 15 litres, so a significant size but fits snuggly on my hips, It looks like Lowe alpine are getting a few good reports in this thread
If you're close enough, certainly, I've bought two great bargain bags from there and an excellent duvet jacket. the wesite is currently saying they have dreamcatcher 1000g (down) (last years model) reduced from 200 pounds to 130. Theres always lots of odds and sods to look at in the shop
Like John says, if you camp careful then down is all you need. Ask yourself 'how many times have I woken up in a tent with a sodden sleeping bag?' If the answer is never or perhaps once then go for down. If you answer'quite a few' then buy a new tent and still buy a down bag. Go to the magic mountain web site for bag bargains of the ME kind.