honestly, i am maxed combat and i still use bandos, i have been since pre-eoc and i don't really understand the tier stuff either, but whatever i am doing is working.. so.. it's really up to you.
It's the highest level power armor (dmg boost) that is non degradable. So it's your best investment imo. Unless you're a hardcore PvM prod, GWD armor is sufficient for slayer and nearly all combat.
Bandos is pretty much the Dragon of the old days - Reliable and efficient. Only against the Uber bosses would you need something better, and the only things better are degradables. ~10m is not bad tbh
