SS is much stronger than al. This lets the manufacturer make SS prop blades thinner than AL props, which reduces drag while it's spinning in the water.
As a "general rule" SS will go a little faster if all else is equal-and it rarely is. You can pull a stock AL prop off of a brand new motor after getting a baseline, slap on your favorite SS prop with the same pitch and it might help or it might hurt. These smaller motors are more sensitive to propping than, say 90hp and larger motors. 4 stroke engines are more sensitive to propping than 2 stroke motors are if they are the same HP. The "early" (mid 1990s or so) 4 strokes even more sensitive than these newer ones. Thus it's really imperative to get the RIGHT prop for your small 4 stroke motor. Generally speaking a 2 stroke motor can "get away" with a mismatched prop more than a 4 stroke would. Notice I said "generally"....not every motor is the same, and obviously not all props are the same either.
it's also possible to take a run of the mill SS prop and send it off for blueprinting, and pick up a mph or two or three. But when this is done, the blades sometimes get thinner and more sensitive to bending and/or breaking (yes you can break an SS prop).
Deflection is a non-issue on props smaller than about 12" in diameter and thus about 75hp and smaller.
Most aluminum props that come factory installed are a good compromise between speed and holeshot. If you're not a racer, usually the stock factory installed prop will do everything you need.
Some factory installed stuff is better quality than others as well. I've seen brand new suzuki's break a blade off at a hint of a strike with the equivalent of a stick in the water; but circumstances are different for everyone, everyone's motors, and everyone's operating style. Some guys seemingly intentionally try to destroy good equipment, others can't help it, and some never have a problem. I will say this: Yamaha's stock aluminum "white" props-especially on the 25's-are thin blades and they bend fairly easily. Don't hit anything and they won't bend but sometimes I know that it's unavoidable.