From my experience most ppl who use roshi use 50% of the ninja cause they cant make the 30combo necessar for his chase and if they do reach it they use ninja that are below average atm. Although Roshi is an end game ninja its good to get him as fast as you can. BUT Ay DEVOURS all the Roshis, ive used Ay with earth main and samui darui and i absolutely have destroyed all the roshi lineups ive been against. So for now i would recommend to get Ay cause that will give u the edge for now and make better progress than you would have done with roshi. I hope i helped in the slightest