Righteous and righteouser? Two types of servant explained

The third mishnah in Avot features the controversial teaching of Antigonus ish Socho that, when serving God, a person should be like a servant who works for his master with no expectation of receiving a reward. There are many commentaries on this mishnah, but I came across one from the Noam Elimelech (on parashat Terumah) that was quite new to me. In short:

An analytical device often employed by Rabbi Elimelech of Lizhensk involves the explanation of contrasting clauses in a teaching by postulating the existence of two or more classes of tzaddik who function at different levels of righteousness. He uses this device to explain the difference between the servants in this mishnah.  

The first type of tzaddik is meticulous in his performance of mitzvot and in the avoidance of averot: he serves his master through punctilious observance of every last detail. Yet, for all that, he still lacks the highest degree of devekut, of cleaving closely to God, and of desire for Him. He must await his expected pay-off in the World to Come. 

The second type of tzaddik is equally dedicated to serving God, but he experiences such a high level of happiness and excitement at being able to serve Him, and such a lofty level of devekut, that he feels that he has already received in this World the reward that was coming to him in the World to Come and therefore expects no further reward. It is to this second class of tzaddik that a chasid should aspire to belong.

A good message, but a challenging one for anyone to live up to, I think.