I hate to have to say this, but I'd select the last one--the medium warm brown color. It's definitely the most classic of the four you've shown. The first two are so very dark and will show every speck of dust. The very dark floors were a fad a few years ago, along with "espresso" colored cabinets. The third looks to be a clear coat, which is good for a casual beach house ("Beachside") but doesn't have the richness of the fourth one.

How much is the upgrade? It sounds strange, since it's just a different color stain. Not sure why they would have to charge more--the labor is the same.

I prefer a very light coloured floor. And in a condo with only one wall of windows, the lightest option would be a natural suggestion anyway. I chose Beachside because I like the look of natural wood (or very light). It will help a dark condo stay light and airy.

As soon as you work a little darker, it will cause the majority of the condo to turn 2-3 steps darker.

If you had a condo with 3 walls of windows, you could easily work with any of the dark or mid-tone products and not worry about how much light came in. But your condo has only 1 wall with windows. That naturally points me towards the lightest floor possible.