For me, any guitar with HD28 in the name is likely worth giving-a-try. These Martins are top-of-the-line boomers. However, the Gibson AJs have their own appeal and sound. Much of it boils-down to what you want and feel attracted to at this time. Would not be an easy choice for me. I already own an HD28 and it's a superb instrument. The upgrades for the 28 truly do make it even more exceptional in many instances. On the otherhand, my gigging partner has a Gibson AJ, and I would die to own that guitar. It's very easy to play and it's the kind of guitar that I hate to put down. Tough choice. Have fun making your decision.
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Gibson J100 Walnut, Gibson Hummingbird, Gibson Dove, Gibson J45TV, Gibson AJ RW, Gibson AJ Koa, Gibson Southern Jumbo, Gibson J15, Martin HD28, Alvarez MFA66SHB and many other guitars, banjos, mandolins, dobros, dulcimers, and strays.