Experience, education and a good reputation are signs of a good arborist. Don't rush into a decision because of discounts offered if you sign an agreement now. Be sure you understand what work will be done for what amount of money. It is not generally a good idea to pay in full until the work is completed. People who aren't competent arborists may solicit tree work after storms, seeing an opportunity to earn quick money.
Storm damage creates high risk situations for both workers and property.
Legitimate arborists never ask for payment in advance. It is generally wise to get more than one estimate and let each contractor know you are soliciting for other bids. A conscientious arborist will never use climbing spikes except when removing a tree. Climbing spikes open unnecessary wounds in the tree that could lead to decay. Good tree work is not inexpensive.