This is a bit of an odd questiion, are you thinking it would save money to buy a 6 screen instead of a 6s?
The short answer is that it won't work. The 6 and 6s screens are sold as display assemblies with the glass and the LCD fused together with optically clear adhesive. The LCD and digitizer connectors are different between the 6 and 6s screens, as is the entire architecture of the screen--including force touch on the 6s.
if you're motivated by cost, your best option for 6s cracked screen is the Apple Store screen swap or mail in repair. As long as you have no frame da
age the cost is $129 for 6s screen change. At Apple the cost for the 6 screen change is $109. If an Apple authorized place is not convenient for you, you'll find that a quality independent repair shop that uses original quality parts will be much higher for the 6s than the Apple Store since the parts cost on a good aftermarket 6s screen is very high. no one will be able to put a screen wirh its connectors made for a 6 onto a 6s. Your best bet for this repair is the Apple Store.