Having an embarrassingly large collection of sleeping bags, I'd recommend down, unless there is a high probability of getting the bag wet. This shouldn't be a problem if it's in an Ortlieb bag or similar. They are more expensive but last much longer, pack much smaller and weigh less.
With down bags the price reflects pretty accurately the weight/warmth ratio. Have a look at Peter Hutchinson's bags (
http://www.phdesigns.co.uk/) - they are absolutely superb (I have three!). He also has a piece on down vs synthetic (
http://www.phdesigns.co.uk/techdvs.php?) although it has a bias towards mountaineering use. If you want something really small and light his Minimus bag is fantastic, although if you want a slightly more versatile bag I'd go for 400g of down and a full-length side zip. Go for less down if weight is critical or you are a warm sleeper, and take little notice of temperature ratings - they are only really useful for comparing within a manufacturer, not between.
Although a huge fan of Buffalo clothes (I was wearing mine on Ben Nevis at the weekend) I really wouldn't recommend the sleeping bags unless you are expecting to sleep in a puddle, or worse. The great benefit of Buffalo is quick drying and warmth when wet, not weight or pack size.