When all the other questions are answered trust is what you are left with. In most cities you will have a choice of professionals to choose from and this is good. You must trust the person you select as the quality of your immigration application is so important. Check out the fees charged. If someone is quoting something really high or really low ask yourself why.
Most professionals charge within a common range.
A lot more is not necessarily better and a lot less is definitely not necessarily better. Ask for references. This is an important question. Alan Davies is a highly experienced professional. This is a national designation and ensures that he operates with the highest integrity and maintains his knowledge level up to date.