eff

tell us about your main floor furnace-brand, model,size and size of the main floor.

Generally speaking, I prefer a separate system over a zoned system. However, there are zoned systems that work fine assuming you have the correct size HVAC equipment and your dealer has good experience with designing ductwork and installing zoning contols.

will the finished basement be used yearround or just occasionnally?

IMO

1) First off, a heat load has to be performed to make sure that the furnace output is at least equal to the heating load of the main plus the basement. If not, you need a new furnace.
2) Second, even if it meets that fundamental criterion, a dual temperature controlled hot air system is either inefficient or fairly complex and costly or are too dependent on the installer expertise.
However, if you aren't too particular about maintaining the basement temperature precisely, you could opt for balancing the flow with manual damper adjustments and keep your existing two zones intact.
A separate furnace is a good option if 1) cannot be met or you want precise basement control and don't trust that a sophisticated damper controlled system will function properly according to the contractor claims. Or, you go the manual damper controlled way and if you aren't satisfied with its performance, then you could, at a later date, either go for a new furnace or controlled damping.