My name's Kel.
Is the new space completely separate from the existing -- say by a doorway?
Or is there a large portal where new space flows into old space?
Is the new work going to be woven into the old work or is there a transition between old and new which is say perpendicular to the lay of the floor?
have they specified who the manufacturer is for their products?
Are number of finish coats the same?
Is moving furniture and dust control included in all bids?
Have you verified they're all the same size companies in terms of sales of number of employees?
What costs A LOT and takes A LOT of skill is weaving new work into old.
There's little of that.
The GC is blowing smoke.
There's NO WAY 'his guy' can do a quality job for less than your lower bidder.
The low bidder is too low for this work. His prep has got to be leaving out a step of cleaning and he's got to be doing less finish coats.
The high guy is off the charts. Is clearly quite busy and doesn't NEED your job.
Concentrate on your middle guys. Ask to see a job similar to yours. Talk with clients. (I always offer this. Any real professional won't be offended. It confirms to me how serious you are and you want high quality work.)
Ask them to clarify their specification. What exactly they're going to do and which products they're going to use.
Are they both including removing and resetting base trim or base shoe?
Are they including new trim?
How close will their match be?
Are there differences in warranties against cracking, heaving and discoloration?
Have I answered your questions?
Hope your project turns out well.