Height changes the job
A small defect beside a high chimney can need scaffold.
The quotation should describe the work
A useful quotation should say which part of the roof will be worked on, how it will be reached, what will be done and what materials will be used.
Check that the quotations are for the same work
If two quotations are very different, ask each roofer what they think is causing the leak and exactly what they intend to repair.