Its not the brand of the microscope that makes the microscope, but the grade of objectives that you mount onto it. In addition, both manufactures, as well as all other laboratory grade manufacturers such as Leica and Zeiss, make the same grades of objectives: achromat (base, cheap), fluorite/semi-apochromatic (mid-tier, more expensive), and apochromatic (maximum resolution and quality, super expensive). A full set of achromats (2X - dry 60 X) will run you about $2K with either manufacturer where as the same set at apochromatic grade will run you ~$12K.
As far as which company you go with, its all personal preference. Olympus likes to tout its ergonomic low-stage microscope and accessories. Nikon doesn't sell a low stage microscope, but no one that has used their scopes, including myself, has complained about their hardware. Both manufacturers also make LED light sources that can quite literally last you an entire career. Its all about what you want and how much money you can invest into objectives.
Hope this helps.