I'm not a salesperson, but I'm very familiar with microscopes since my department purchases them based on my recommendations.
I don't advocate one company over the other, but I do advocate the grade of optical objectives you mount onto your microscope. Plan Achromats are the entry level objectives, Plan Fluorite are mid-level semi-apochromatic lenses, and Plan Apochromatic lenses are top of the line. All the 'Plan' means is that the projected image is adjusted for complete flatness (planar) for the entire field of view. You'll notice a jump in optical quality from plan Achromat to plan Fluorite; sharper contrast especially at the higher magnifications and richer colors. The jump in quality from Fluorite to Apochromatic is debatable depending on who you talk to. What isn't debatable is the price. A full set of Achromats (2X-60X) runs ~3K depending on what contracts you have with whom. The same objectives at the Apochromatic grade are ~12K. As an attending pathologist, I wouldn't settle for anything less than Fluorite grade objectives.
As far as frames, that's user's preference. I've heard arguments every which way of why people like Olympus over Nikon and vice versa and why they picked a certain frame over another. I will say that the only thing you have to watch for is that Nikon makes a 1X and Olympus makes a 1.25X and they need special condensers to function properly. So if you're entertaining the idea of having one of those objectives, make sure the frame and condenser you pick out are compatible.
Anyways, hope that helps.