OK, I looked at the Spec for your unit. I do not normally deal with Valor however I used to install the Legend G3 and loved the unit.
Yours is unique because it allows for an adapter from standard coaxial to colinnear venting. The co linnear is usually with an insert (such as the G3) You will most likely not notice the diferance between the 2 vent systems once installed. I would go with the co linnear system (2 3" units) in stainless (just because it lasts longer). Thare is no real benefit to the coaxial vent. BTW you use the words "direct vent" meaning how the units is venting through your house (directly to the outside) but Direct vent is used in the industry to mean a balanced flue system that may only draw air from the same relitive location as the exhaust. just an FYI
Now, ask installer 1 and 2 what they plan to do with the chimney when the run the liner through. If neither of them suggests additional insulation at the top(mininum) and either of them wants to draw air from someplace as opposed to running both liners all the way up drop them or let them know it is expected that they silicone insulate and terminate properly. Silicone should be run betheen the flashing and the masonry at the top. This will help in a high wind situation and also slow drafts.