Which would install on a stock '15 RGS stock/no amp? They both cost about the same.

 

Neither.

 

What, then, would YOU use for this simple speaker-only swap?...and why?

 

What my friend HAZE is trying to tell you is it does you no good to swap any speakers on your new bike if you are not going to amp them. You might as well stay with the stock speakers that is set up with your HU. Amp is a must have to improve sound. You may notice a slight difference in the garage with the new hog tune 6.5's but again. As soon as you fire up your bike and pull off you'll be right back to square one.