[Mike56]

My first guitar was a Yamaha FG700s and I still own it. As CharlieStrings said, the FG700S is a large dreadnought while your FG150 is equivalent to a 000 size. And I can confirm the FG700S has a solid spruce top. The Yamaha FS700 and it’s replacement the FS800 are closer in size to your FG150.

I’ve never played a FG150 so can’t compare sound, but can say the FG700S sounds far better than you would expect given the very modest price. But your 50 year old FG150 probably sounds pretty good after 50 years of play.