get the 155... at 9din you'll be close to maxing out your din setting which is not ideal (for a 12din binding).
It is true that maxing out the range on bindings isn't as good as getting a binding with a higher range. If you have a din of 12 and crank it to that, it will not bust on you, but it is causing unusually high stress on the spring and wears the binding down more quickly.
To answer the question: snag some 155s. The only reason I could see to look hard for some 120s is if they were in a color you wanted and the 155s wer enot. As is though, the 155 is a better overall binding for a hard-charger, and you won't use the bottom of the din range, but could possibly use the high part, so why not?
While a binding is designed to operate at all the DIN ranges on its scale, it's not advisable to max out the DIN. Reason is because the spring is nearly compressed fully and there is no more room for compression. Always safer to release the tension in your springs in the off season. Springs do wear out and release settings do change if you leave them compressed.
You are correct in saying a higher end binding will be stronger. They have to be to handle the forces associated with higher DIN settings!
From a safety standpoint, I prefer to ride at a DIN that will actually release. I crashed about 3 years ago and blew my knee out on a forward twisting fall. Was riding on Rossi Axial2 140s on Bandit B2s at a DIN of 9. Problem was I was dead tired and while the binding did release, my muscles couldn't hold up and pop went my knee.