well i think starrk and Grimmjow are craving for something different. Starrk craved for company while grimmjow craved for power(but hd company).I honestly think stark had a better charcter formation than grimjow. I mean he is basicly the king of espada besides aizen. He would weep for his comrads' death. I honestly think grimjow was an ass that got lucky by making it as far as he did. He didnt want to be the "king"but given his natural abilaties he was named king. These two seem opposite to me, grimjow never truely came accross hardship like he did grimjow wanted the death of other arancar fpr his power, but all stark ever wanted was a friemd, and for him thats were the true power resided.
since stark is already powerfull, all he needs is a friend who can tolerate the spiritual pressure which means the other espada.
i doubt starrk would weep for comrade's death. he will just need somebody else beside him.
Grimmjow is just like kenpachi and vegeta who are ambitious for power and is more of a survivalist.
so both are totally different so who is the better one is difficult