Synthetic - bulkier & heavier but cheaper
Down - Less bulky & lighter but more expensive
Depends what you're after really mate, what sort of budget are you thinking ?
If you want to treat yourself then I'd recommend the
Alpkit bags, I've got the Pipedream 600 and can't rate it highly enough. I'm also down on the waiting list for the 400
They get them made in batches which sell out really quickly these days so you need to get yourself signed up for their newsletter.
If you can't stretch to a down then I really liked my Snugpak Elite 4 which I used for winter camping for a couple of years. Well made and just down the road from you, try and call in to their factory shop for a few bargains