There is a google chart somewhere that compares effective str lvls between getting dditional str lvls and unlocking new prayers and iirc getting 13 prayer instead of additional str lvls is better after like 80str or so and 31 pray is better than getting the same cb lvl from training str after high 80s or so.
you dont really need extra hp lvls so higher prayer is generally better, especially with protect item.
just check a cb calc with your acc stats and your maxhit and then check how many cb lvls you gain with either unlocking the next boosting prayer vs training str to get to the same cb lvl as you would be with the next boosting prayer and compare maxhits