Having kept both, blyxa japonica and dwarf sag look quite a bit similar in the tank, even side by side. The difference, as has been mentioned, is one of shooting runners and growth. Blyxa is an easy to manage plant. Dwarf Sag is anything but. Not only will it shoot out constant runners in high light, the roots are absolutely massive and will suck nutrients away from any neighboring plants, in some cases even killing those varieties who aren't equipped to compete. Try to uproot them? You're taking half your substrate with you in that case. When I think of my ultimate aquascaping nightmare from a maintenance standpoint, I think of a high light tank featuring glosso and dwarf sag. The combination makes me shudder. All that said, if you need a quick scape, or you need to fill in some areas on the quick, you couldn't pick a better combination. So, just like with so many plants, it all depends on your goals.