It all depends on the height of your beans and how bad they will shatter. With anything you have to look at return on investment. An air reel is said to save you up to 3bpa. At $8 per bushel you would save $24 per acre. The shorter the beans the more you would save with an air reel. So at the average of $10k cost to add an air reel you would pay it off in a couple years just on the savings from beans at 3bpa. If you grow taller beans, chose the right variety, and plant on 30" rows you should get by without an air reel because the regular flex head should be adequate.
I have the same machine as you and bought an older 974 macdon flex draper with an air reel and couldn't be happier in beans. The feeding of a draper is worth the extra money but I cover a few more cereal acres to help justify it. I also solid seed beans with the air seeder on 7.5" rows.