Two of the worse models of eotech. The 551 is the "smaller" version of the 552 which is a honking beast on top of a rifle. The 552 uses AA batteries which have been known to jiggle and lose connection during sustained fire. Although AA batteries are much more common than the 551's N cell batteries, which are far too underpowered and go dead very quickly. You can find the N cell batteries at Walmart in the camera section, but walking into the local 7/11 and buying an 8 pack of N cell's is out of the question. Given the choice of those two terrible Eotechs, I'd go with the 552. The 552 I had had a much much better battery life than the 551 I had afterward.
The XPS line of Eotechs are far superior than anything they've put out before. Go with an XPS. They can be found fairly cheap in used classifieds.
The XPS line of Eotechs are far superior than anything they've put out before. Go with an XPS. They can be found fairly cheap in used classifieds.