I dont know about the sigma, but recently I tested a D-15 and the sound was awesome.
Great mellow sound.
I will be buying it as as soon. If sigma is also a solid mahogany then go out and test it.

: I someone who is looking to by my first guitar.
: I have narrowed the choice down to a slightly used Sigma DR-28 and a slightly used Martin D-15
: I would appreciate any suggustions on which I should by.I just got a d-15 use it in shows w/ fishman pre-amp sounds great and has a punch. I hve never played the sigma so I can’t comment. Buy it, its killer go to music 123 I got one for 600.00 Plus it came w/ hardshell case

Well, I don’t know about the Sigma, but I have a d-15 and I absoulutely love it, i can’t put it down, it is a solid body guitar and is a martin so that is quality unmatched. Pesonally go with the d-15 you will not be disappointed