Prop Selection Johnson 70EL77

[Cherokee-Runabout]

I'm sure that this question had been asked a thousand times on this site but I would like some confirmation if possible. I have been running an aluminum 13.75 x 15 prop (which is the same size as the factory supplied) on my 1977 70hp Johnson on my 1977 Lund Shellstar. The boat without passengers or motor weight roughly weighs between 900-1000 pounds. My top end with two people is about 31-32 mph. Currently do not have a tach hooked up although I understand that it is probably one of the more important things here. I sometimes go on the river and after any sort of rain there tends to be a lot of tree trunks, branches, firewood, etc... floating in the water. Unfortunately when trying to keep any sort of plane around dusk some of these smaller pieces become almost invisible. So is it worth it to keep spending $150 aluminum props or should I take the leap to stainless? I've heard that the stainless do well against branches or other random chunks of wood? (rocks don't seem to be an issue) If I do choose stainless should I be worried about stripping gears if I hit a decent size log? And finally because of the added weight should I move down to a 13 pitch? ( also hard to tell for certain without a tach) Thanks for any info anyone may have, just trying to get a better idea/check what I think I know.