General rule of thumb on big motors is SS will let you step up in pitch by 1". This may not hold true for mid sized motors - dunno. The 21 you've got can't be measured with a ruler, the 21" desribes the rake or cork screw in the prop and can only be accuratly measured by a prop block. All else being equal, a SS prop is more efficient due to less flex in the blade translating to less slip and more power being put down. You should also see a quicker hole shot.

As far as which prop to go with - go by a good prop shop...one that specializes in outboard props. Most of any size will let you try before you buy. As G said, never under prop a boat and over rev your motor. Not sure on your motor, but most Yammie 4 Strokes want to be propped at 5500 which is mid point of the recommended range. There's the sweet spot. It sorta like having a 363 Stroker with a 1 speed transmission. You've got to find what works best for the largest range of how you want to run. Prop for top end, and hole shot stinks. Prop for hole shot or load carrying and top end stinks. Prop for handling in turns and one or the other or both stink. Shoot for the middle in most situations.

The Prop Shop in Jackson will let you try before you buy, but given the quantity of boats similarly rigged to yours, a high volume G-3 dealer could prolly tell you without thinking which prop would be a good one for your rig.

Wannabe...