definitely recommend the vaude models. Far more comfortable than others I have borrowed and DS felt secure when he was smaller, yet could see plenty. I have only stopped using it now as I am pregnant and was having dizzy spells. DS is 2.6.
Look for one with a kick-stand, so that it will stand alone, and nice padding round the adjustable straps.
We got one from ebay for about £40 second-hand but will buy a new one (or newer!) as we got so much use out of it, and we live on a farm so it's pretty kicked
We have the Vaude Jolly Comfort. Tried the Ergo but found it too hard to adjust straps, whereas with the framed ones, all the adjustments are done before you lift them on to your back. I stopped carrying DS in the Vaude this May when he weighed around 25lb, but only because it was making me too hot & I found my shoulder bones (at the top!) were hurting. I'll start using it again in the autumn when a) it's colder and b) I'm wearing a coat (so more padding on my (bony!) shoulders). Would highly recommend - DS adores being in it & it takes seconds to put him in it & get out the door!
I have a Kelty Kids backpack. The design is great, very comfortable for the adult to wear and the child was very secure inside. It had a kick stand too. I'm selling it on ebay at the mooment - unfortunately ds2 took an instant dislike to it and we only used it once! No need for one now as he is 4 and it's been in the loft for ages.
We have a Little Life one. They are a little cheaper than bush baby, but the same. We have used both (Bush baby and Little life) and both are good. We went for one without a removable carry bag on the back (ours just has a zip up section, but not removable). It was a mistake. Get one with removable back pack if you can stretch to it.
This is what we have. They have the one with detachable day pack on the same site.