Hi Bien, I can't really answer your questions properly, because I have never bought diamonds or coins. However, having accumulated quite a few diamonds by collecting them through toolboxes, the mine and viewing trailers, I can tell you how I do spend them now. I hired Tom for a day, so that I would be offered a discount for hiring him for three days or more. Now I occasionally hire him for three days to stock up on jam and shovels. I also used diamonds to speed up the last stage of the fishing area and the lobster pool, just because I was i patient. Sometimes I use 1 diamond for an extra spin on the wheel, but invariably I find it's not worth it. I also used diamonds to buy some of the Halloween decorations, because I really like them and because I felt there wasn't really anything else to do with the diamonds. I have never used them to speed up upgrading my barn or silo. I'd be interested to hear how other people use their diamonds.
Edit: forgot to say that the main way I have spent diamonds is to add extra slots to production machines.
Chumleyboo, level 72.