No, the House of Dimir is not better than the Grull Clan, because the Grull Clan is bigger and more powerful. The House of Dimir is better with secrecy, but secrecy alone is not the appropriate measure. The House of Dimir is also more effective when there are not a lot of characters fighting, which means it's not really effective.
Gruul was once maintained wild places. They themselves were wild but very noble. But they
were soon overran. Now house of Dimir is different. They did some unsavory things. Espionage, burglary, smuggling and other illegal services. Those type of deeds just puts them as an inferior to the Gruul Clans.