in a true high hp setup were you go large cams.. the cost of the 26 and 25 still end up being the same as you have to upgrade the lifters to tomei items in the 26 head with 10.25mm lift cams and larger.. so thats 400odd.. and the rb25 solid lifters made by tomei are about the same price.. so really there is f**k all difference
the only real advantage i see from using a 26 head over the rb25 head is the fact that the 26 head setup correctly can handle 11.5mm lift were as the 25 can handle about 10.2 max(from memory)