I prefer OA, mainly because I'm used to working with it and it is inexpensive to use. I see Evaporust as the product for smaller jobs and jobs where bleaching paint is a concern. I use OA for just about everything else.
A few warnings:
-Evaporust is prone to softening and degrading period decals.
-OA will bleach certain period decals, reds, and gold paints.
-OA will form a stubborn, yellow crust if mixed too strongly or left in too long.
-OA should be pH neutralized (baking soda in water will work) in hard-to-reach places that you won't be cleaning any time soon.
-Both will lift compromised paints. Evaporust will do it faster, but both will do it.
-Both (but OA in particular) will remove certain kinds of surface plating, such as galvanizing and CAD plating.
-Both will remove bluing for wartime type finishes.
-OA can be mixed to your preference for weakness/strength.
-Both do better in warm conditions: warm workshop, warm outdoors, etc. OA benefits from use of warm/hot tap water in the mix.
If you're cleaning the inside of a frame, I'd follow up the treatment with something to neutralize the pH of OA, and then a good preservative like Frame Saver, Fluid Film, or even WD40. Let the excess preservative drain out before rebuilding. With Evaporust, I'd just do a simple rinse and preservative treatment. PH is less a concern with Evaporust (but a lot more expensive for big jobs).