New brakes. 9" or 11"?

In my opinion if the Jeep has the original Power plant and you use  the jeep for what it was designed to do ,45 to 50 MPH back roads traveling , The stock setup is perfect. If you are going to upgrade to a V6,  up grade the brakes.  The trouble with upgrading is it never ends, just buy a new wrangler and be happy with a all aluminium engine and plastic grill.

Go with the 11s. The 9s are a pain to adjust, and even when they are adjusted right they don't work great. The 11s are much nicer to deal with, and makes stopping much safer and much less dramatic. I've had to panic stop with 9in brakes before. It is not a fun experience.

ditto rrrrrr9

I know that the 9" are fine when they are adjusted properly, I think the prospect of self adjusting brakes is what appeals the most too me.  If all my drums are servicable as is I will probably stick with the 9" brakes.  If I need all new drums then I dont think it would cost much more to upgrade to the 11".  If I have read enough about it all I need to source used is the backing plates?  Everything else is available new.

The 11" backing plates can be sourced from $25 to $50 each; fronts are easier to find than rears.
Fronts can be used on the rear if you don't want to use a cable operated parking brake at the rear wheels.
1971 and prior CJs used transmission parking brakes. so as you can see using front backing plates on the rear is a viable no brainer.
All of the 11" shoes, cylinders, and other components can be purchased on-line and rather inexpensively and delivered with free shipping to your door. Drums are climbing in price though.

Backing plates can be sourced from 72-75 CJ Dana 30 or 44 axles; these are the most common donors as you find these axles still in aboundance; early Wagoneers and 72-73 Commandos are donor sources as well.
Dana 30 used axles removed from vehicles can be purchased with brakes for anywhere from $50 to $150 bucks; take the brakes and part out the rest to recoup some cost.
Post Office DJs, early Fords and Mercurys, the list goes on and on for brake donor vehicles.

I did the 11" conversion right after I bought my Jeep.  My Dad's friend had the 11" conversion and he has been a Jeep guy for 50 years or so.  He recommended it, so I did it.

4 wheel drum brakes.  Man do they stop great!!  Almost too good!!

Is there anywhere online that I can source the 11" backing plates? 

I've upgraded to the self energizing 11ers ten years ago on my first early CJ5, swapped those axles over to my cj3b that I'm driving around now. It's a worth while swap, driving around in city traffic ya never know when they will save ya from purchasing a new cars backside when a idiot wants your lane.

Jeeper50 hit the nail on the head "self energizing" is the magic system on stock cj2a brakes you have two anchor points top and bottom the shoes only use pedal pressure to stop the jeep with self energizing brake there is one anchor pin at the top as the front shoe comes out it contacts the drum and forces the rear shoe by pivoting on that one anchor pin into the drum multiplying the braking force (energy) stopping the vehicle better. Alot of drum brake cars came with 9 and 10 inch brakes a 66 SS 396 Chevelle  came with 10 inch brakes on a 3500 lb car.