Alison ,
Denise & Stewart are absolutely right, but to add to the contributions I would say that the considerations should be;
1. Budget, buy the best quality you can afford.
2. What do you want the awning for? I know that this sounds a bit strange, but if you only want one to keep the weather out & keep your wellies in then there's no point in buying a full awning, conversly, if you need the space then ther's no point in buying a porch awning, personally we have 2, a porch for weekends & a full for longer stays.
3. Weight, if weight is an issue, then can it be carried in the car, if not then light weight poles, fibre or alluminium are a must.
4. Colour should be your last consideration.
Finally, one last tip, what ever you end up with, study the instructions (if any, may not be with a second hand one) work out how the poles go together & do a dry run fitting them together, if you have space at home or at your storage location, put the awning up before you go away, (saves a huge amouint of embaressment on site!) and liberally spray the awning rail with silicone lubricant, don't half make life easy attaching the awning to the van!