Its very difficult to get all the makes in one place.
Little Trekkers (Penistone) appears to be on of few in the area that stocks most brands. When we spoke to them they said they have most in the warehouse and would be willing to get them out for us to try out.
In the end we just got a Little Life Freedom from Cotwolds as we managed to get 20% off and after trying Deuter, Bushbaby and Vaude, Mrs FD and I found it the most comfortable.
Things I would say to look out for are:
* A big carry space to put nappies, wipes, food etc etc in.
* Easy access to external pockets for just stuffing stuff in to easily and quickly, we really glad we got the one we did with easy access external pockets.
* Sounds daft but a mirror or some where to attach one. I know if I go out walking with Junior FD its handy to be able to see him in case his hat has fallen over his eyes etc.
* Some kind of additonal neck/head support. My son went in since 5 months old and still at nearly 8 months likes some hea/neck support after about 45 mins.
* Sun cannopy. Ours came with one, but wouldnt be without it, helps shiled from the wind as well as sun, mainly though Junior FD rests his head against it.
* The Littlelife have what they call an anchor point, basically a hole you can stuff your foot in which keeps the pack stable whilst you put your son/daughter in. This is a realy help if your by yourself.