Martin, being the king of the FLATTOPS has also a lot of O or OOO guitars.
Which guitar is the best to play, and what guitar is better, talking about sound?
Is it similar to a GIBSON HUMMINGBIRD or DOVE versus A GIBSON SJ-200?
Not only do Gibsons and Martins sound different from each other, the different guitar shapes sound different too. I have a long history with both, and here are my descriptions of the sounds:
Martin (generally) -- very clear and woody, vibrant highs as well as powerful lows and midrange, top is more resonant
Dreadnought (D28, D35, etc.) -- most powerful bass, best for strumming but the mahogany ones (D18) are amazing for bluegrass (Tony Rice) and the rosewood ones just sound incredible behind someone singing really well (think Seals & Croft, David Crosby, Steven Stills)
OOOO or M -- large round body but shallower than a dreadnought, notes dissipate quicker so it's great for recording studio, all the folk musicians in the world love this one (Arlo Guthrie, Shawn Colvin, etc.)
OOO and OM -- smaller rounder body, a finger player's guitar, Eric Clapton has several, great for blues because of the balance and highs but the lows aren't as resonant as in the larger guitars
OO -- a little smaller but deeper body, traditional blues guitar of the really old school like Elizabeth Cotton, Goeff Muldaur (yes, I know he's not nearly so old...)
O -- even smaller, clear tones, Joan Baez plays this one
Gibson (generally) -- darker and "full" sound, power in chords, notes are quicker to fade to make room for more
Jumbo (like SJ) -- really loud and bassy and "chunky" too, sound great with internal pickups, think Pete Townsend playing acoustic guitar on "Pinball Wizard"
Dreadnought (like Dove, Hummingbird, older Southern Jumbos) -- most powerful bass, still a strumming guitar all the way, but Jackson Browne plays one man lead on it, listen to his Solo Acoustic album...
LG and similar -- smaller rounder shape, tone very much like OOO Martins, nice highs but lows not as resonant as the larger guitars
What do I play?
Martin HD28 (dreadnought) is my favorite, for folk and acoustic rock and pop
Martin OM28 (OM, OOO) is for sweet fingerpicking songs and jazzy blues
Gibson LG2 is for hard core down the blues my naughty sweetie gave to me
I'm personally partial to Martins because (1) I play with my fingers a lot, and they respond better, and (2) they go better with my voice, and (3) I like the clarity of the sound.