Likewise I wouldn't expect a big difference in saddle height due to going to clipless. It's not been anything I've ever been concerned with. Maybe at most a silly centimeter longer. Sort of like the old Chesterfield commercial long ago before they were banned. (only it was a silly millimeter longer)

I don't know how you should base choices you got and they don't specifically say online. These are for the 58 cm frame.

Saddle rail height min (w/short mast) 69.0
Saddle rail height max (w/short mast) 75.0
Saddle rail height min (w/tall mast) 72.5
Saddle rail height max (w/tall mast) 78.5

I sort of assume that with the they are the min max you can get measured to the BB then you have to add in the crank length and the height from saddle rails to top of saddle.

My 56 cm Tarmac is currently at 73.6 cm between the rails and BB. I'm 180 cm with 87.6 cm inseam. So that sort of makes sense what Giant is showing.

You can simply measure your current bike if the saddle height is to your liking from the bb to the rails. Since your cranks are 170, then for the bike with 175 mm cranks you need to take 5 mm off whatever you measure.

The issue I see is that if the bike comes with 175 mm cranks, then you might be better off with the short mast. If you are going to change them to 170 mm cranks later, then you might need the tall mast as shorter cranks mean taller saddle height.

How much is a mast if you find later on that you need the other size?

I wouldn't sweat the clipless/flat pedal thing though. I'd actually doubt it's the cm more I mentioned. Though again, I've never thought to concern myself with it.