I decided to get an EOTech for my AR but am stuck between two models, the 551 and the 552.  The only difference between the two is the power source.  The 551 has a small battery and the 552 has a AA battery.  Which one has more pros than cons?  Does the length really affect anything?  

Any advice owners of either models have would be appreciated.  Thanks.

Break it down to batterlife. Compare the run time with the cost of the battery. If the difference is insignificant, I'd go with AA because of availablility.
Also, try the optics forum.

I personally went with the 551.  I wanted the versatility of mounting location -- the 552s longer body prevents you from putting it closer to the front of your flat top (at least it did on mine).  N cells are not impossible to find, and although I'll go through many, I don't think I will make up the additional cost of the 552.  The 350 hour battery estimate on the 552 is against Lithium batteries by the way, I believe.  I don't know how long alkalines last.

If you're going with the 551/552 because you have night vision, I'll assume you have a forward extended rail where it will be mounted anyway, so length probably is irrelevant.  In this case, the 552 makes sense.

YMMV

the AA ones are longer than the 123batt. ones, i would get the 552 model,

My decision for the AA version was based on battery life, but also on other considerations.

There could be times when you can't get to a store, the store is not open, or for some reason, N batteries are not available.  

I looked around the house.  AA's are easily scrounged from a walkman, a AA maglight, camera,  and any number of other small items.  Perhaps the batteries would not be new, but I would bet they would do OK in the Eotech for a while.

(Edited to add)  From Eotech's own information, the N Lithiums last 70 hrs.  AA Alkalines give 300 hrs, and AA Lithiums give 500 hrs of use.  This is a substantial difference.

If used carefully, AA Lithiums could go for several months of daily use.

You can store ALOT more N batteries into a pistol or fore-grip or buttstock, than AA's.
IMHO.

I was about to ask the same question in the near future, because an eotech is on my wish list as well. So battteries are the only difference right? I'm not intending to put on NV. I'd go for the 551 for price and looks. The 552 just looks weird to me. Kinda looks like it has a big nose...Anyway for the price difference you can buy PLENTY of D's for when SHTF.

linky no worky [:D]

Lockedon:  

It takes SEVEN sets of N's to equal the hours of operation of one set of AA Lithiums.

It takes FOUR sets of N's to equal the hours of operation of one set of more common AA alkalines.

  Another vote for the 552, batteries are more

 available and have good life. (I keep a spare set in my G-27), no probs mounting on the Beowulf.  jmho..

That wasn't my point though. I simply said that it would be easier to store MORE  N  batteries in your stock, grip etc...
But whatever works for you.