Assuming it is in good condition, the color on the downstairs floor looks fine. It's classic.

As such, I would consider matching width and species upstairs. And I would use stair treads stained the same color.

If the downstairs floor is in poor condition, then you could restain it a little darker (don't go past "medium brown"), but still stick with the same width upstairs.

The downstairs is in the part of the house that you will show off to your guests. Wider widths are considered more upscale/expensive. It would seem odd to have the "fancy" part of the house in the private areas where no one would see them. And it would seem to be a waste of money.

A word of caution: your stairs may have been designed for carpet, so you could have some unexpected expense in removing the carpet there. I would remove the carpet, though, if you are going wood upstairs.

As an update and investment, I'd change the flooring to match through the whole house including the kitchen. That gets you 7-11" wide plank flat cut light non-yellow European Oak.

This option has only become available in the last few years in the US. Only as engineered hardwood can you get a stable product in the wide plank to show the structure of the wood. European Oak does not have the open grain of white and red oak. There are zero houses built today using 3-1/2" wide solid hardwood for flooring.

Many choices including Hakwood, DuChateau, Monarch Plank, Hurst Hardwoods, Armstrong... Everyone has this product, imported from Europe or Asia.

I think your original floor is fine no need to change it the palnk width is also classic and save the money and do the rest the same wood and color .

Upstairs can be wider if you wish, I would try and get it to the same color and of course the same species as downstairs. Stairs will be tougher as you cannot simply add flooring you would need to replace the treads if possible.