Sigmas can be all over the place in quality and sound. The ones to look for have the inlaid name plate and made in japan. I have had two now, the D18 clone was absolutely wonderful and I deeply regret ever having sold it. I now have a DR28 which is something of a D28 clone, laminated back and sides, V neck. Its OK but not a patch on the old 18 model, or for that matter my martins.

I also had a korean one - yuck. Heavy, laminated top, and a transfer logo. Flicked it immediately it was rubbish.

Looks like go for the older you can, and make sure you test it. the good ones are 90% of a martin, for pocket money prices.