First of all, welcome to WFM!!!
My 1st. 318 came with a 50” high lift deck. Unfortunately I sold this tractor & deck many years ago. My 2nd. 318 came with a 46” deck. The 50” deck by far out performs the 46” deck.
Last year I picked up a 48” replacement deck with “Tri-Cycle” mulching Kit. It cost an arm and a leg. More than I paid for the tractor. I mow about 2 acres, and have plenty of trees. With the mulching kit on the tri-cycle, I never need to rake or mess with a bagger in the fall. So for me, the new 48" was worth the extra price.
I must also add, this new replacement deck is extremely quite. I do not hear the deck blades at all when it is running, even at WOT. Normally though I only cut at ½ throttle with very good results.
If you are not looking for mulching capabilities, I would go with the 50” deck. In my experience, it did the best job of cutting.