I borrowed a Backpack to try out when I was considering moving over to WFP. I purchased the trolley at the top of this thread and whilst OK I did find the small wheels a bit of a nuicance especially if you had to take it very far. ALDI then had an offer on the other trolley pictured on this thread. I purchased one and found it much better. So I would go for that one if you can find a supplier.
ALDI had another one the other week which looked every bit as good, and it was cheaper too.