Doesn't the big block have a better valve angle too? 15 vs 18 degrees ?
Guessing mainly size, weight inside and out, bearing size ?
Sbm does have a lot of decent heads that will make enough power to keep most people happy especially at the rpm ranges most want to build in Eg. under 6500 rpm, the differences are less, but the bbm should be able to make 100+ hp over the sbm if both are built with logical parts Eg sbm trick flow vs bbm trick flow but probably at higher rpm depending on efficiencies. But if building both with similar head capability for similar rpm and tq/cid than out come should be similar but then are just really handicapping the bbm.