Hard surfaces require soft soles, and soft surfaces require hard, so combinations of hard and soft materials are better: Felt soles (soft) are improved by small carbide (hard) points, and rubber soles (hard) are improved by larger area aluminum (soft) cleats.
For the best grip it is imperative that your studs DON'T protrude too far so the only thing making contact with the rocks are your studs. The top of the stud should BARELY protrude past the sole material so the sole AND the stud make contact.
If you are adding studs to rubber, it should be placed in a recess of some sort. If it is felt, the worst option are sheet metal style screws UNLESS you make a recess. Otherwise go with Grip Studs or
Guideline style tiny head carbide studs that can be installed with the right amount of protrusion.
FWIW - For comparison's sake using the same felt bottomed Chota STL shoes, over several days and trips over the years, I tried several different studs at a stream where I've slipped with worn carbide studs and even a carbide tipped wading staff needs to placed carefully.
Knowing that worn carbide studs were risky, I experimented with other options including Russell Wading Shoes with real wool felt.
On my Chota STL's shoes that have stud posts with minimum protrusion I tried:
New Chota hardened steel studs.
New Orvis carbide PosiGrip studs
New Kold Kutter hardened steel studs
New Gold Ice Screws (hardened steel)
New Aluminum sheet metal screws
New Goat Head studs
New Grip Studs
New Guideline studs
The best grip I got hands down was with brand new Kold Kutter or Gold Ice Screws which diminshed slighty by the end of the day. That's becuse they are VERY sharp and cut/bite into the rocks better that a carbide "point."
The second best was plain the REAL WOOL felt on my Russell's
All the carbide studs when new were pretty good, especially the Orvis PosiGrip but not better. However, the bite lasts MUCH longer than hardened steel screw-in studs.
The aluminum sheet metal screws were OK, but surprisingly not as good as the Kold Kutter or Ice Screws, probably because they dulled FAST and weren't large enough to grip like aluminum bars or disks.
On the day I slipped at this stream with some worn Orvis PosiGrip studs, I landed HARD on my forearm and could have broke it or my elbow in the process or hit my head on a huge rock behind me.
Even worse that that, I could have broken my rod or reel...
😉
So for ME at this particular stream if I am not using my Russell's, I'm screwing in a new set of Kold Kutter or Gold Ice Screws in each day. It is a bit of a hassle, but they are cheap enough and I'd rather spend a few bucks and minutes and know I am getting the best grip possible at the worst places.