PEI has to do with abrasion resistance, not slipperyness. 3 is fine for a bathroom where the users will mostly be in slippers or barefoot. It would not be fine for a powder room next to the mudroom where grit and dirt could cause wear and tear.
For slipperyness, you want to look at the Coefficient Of Friction number. For a wet area, you want a .5 for an occasionally wet area like a bath. However, even a tile that does not meet that standard can have more ''grip'' when it's in a small format that will give you more grout lines to help with traction. So, a very slippery glass tile in a 3/4" format is frequently used as a shower floor, and most people have no issue with it.
But, if you have balance issues, or are designing for ADA, it's always better to go with the higher COF tile. Yes, that does mean it's a bit ''grabbier'' to dirt, but that's a minor tradeoff to potentially breaking a hip because you slipped in the bathroom.
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