Hi there.
I put a theoben strut in my xocet xs and it shoots like a different rifle. From cocking it for the first time after fitting the strut i noticed a huge improvement in smoothness and much less whiplash on firing. You also have the added fact that a gas strut will last a lot longer than a spring no matter how good you buy. A definate recommend as far as i'm concerned. The only advantage that i can think of with the 3-9x50 over 3-9x40 is the 50 mil lens will gather more light into the scope. Hope this helps.
DZ