While all people performing tree services in Maryland should be properly licensed by Maryland and insured, they may not necessarily all be well versed in best practices.
Certified arborists have advanced training in tree biology and health and are credentialed by the International Society of Arboriculture. Most larger tree companies have them on staff.
You should be able to call local tree firms and ask for them to come and give you estimates, then compare, including your personal assessment of their professionalism.
If the trees are important to you and you can't stand the thought of losing them, we'd tend to go with the arborist if you can afford it.
Talking with neighbors and family who have had good work done is also a good idea.