Lower ron shouldn't do any damage to your engine, but it will run less efficiently and you will get a drop in performance. In a high-compression high-performance engine, cylinder conditions might let the lower ron stuff ignite too early. In extreme cases this can cause severe damage - because instead of a smooth rise in pressure as the piston is traveling down anyway, you get a pressure wave slamming into the piston as it's moving up. So basically, 91 should be fine for normal driving if you really want to use it.
For what its worth, I would stick with the 95. My car much prefers starting on 97 ron than 95, so I dare not think what it would be like on 91 ron.
Obviously, you will get more miles for your money with 91 ron since it is cheaper, so you can get more petrol for your money.
Isn't it reccommended that we run it on 95 ron anyway?