EOTechs -
550 Series -
551 - N Batteries, NVG Compatible
552 - AA Batteries, NVG Compatible
553 - CR123 Batteries, NVG Compatible
554 - AA Batteries, NVG Compatible (Same as a 552)
510 Series -
511 - N Batteries, No NVG Modes
512 - AA Batteries, No NVG Modes
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You're unlikely to find 553s or 554s for sale anywhere. The 553s aren't quite out yet, and the 554s might be stolen.
If I remember correctly, the AA versions have about twice the battery life of the N battery versions.
My preference is for the 512.