I have my house up for sell, but it has one major flaw....it's
close to a busy road. Now upstairs on one side overlooks the road
a bit and this is clearly visible from two of the three bedrooms.
Do I show any potential buyers the upstairs first or the downstairs
(which is nice and spacious) ?

Of course you show the downstairs first; that's where they come into the house.
As a retired realtor, I can assure you that buyers will expect to pay less for
your house than another one just like it farther from the busy road. And yours
will take longer to sell.
Pat