The root κοπιαω (κοπιωντες is the present active participle form) does include both the idea of 1) the passive state of being weary from labor and 2) the activity of hard labor, sometimes to the point of exhaustion. I think the difference between those two ideas is more a difference of perspective than of essential meaning.
In a strictly grammatical sense, I think we should prefer the ESV rendering of κοπιωντες ("who labor"), although the NASB does not damage the sense of the verse with its rendering of "who are weary."
Κοπιωντες is actually parallel to πεφορτισμένοι (perfect passive participle from φορτίζω, frequently translated "are heavy laden" in this verse). Both participles are substantives, and they function as the subjects of the main clause: The ones who κοπιωντες and the ones who πεφορτισμένοι are the ones who are to "come."
Because κοπιωντες is an active participle, a literal translation would use an active verb. This pattern appears in the ESV "who labor," KJV "ye that labour," and NKJV "you who labor." The NASB actually renders κοπιωντες as a passive.
Because πεφορτισμένοι is a passive participle, a literal translation would use a passive verb. This pattern actually appears in the KJV, NKJV, NASB (implicitly), and ESV.
The crucial difference between the ESV and the NASB is that the NASB interprets κοπιωντες as a passive to smooth out the parallelism with πεφορτισμένοι. Notice the rendering: "all who are weary and [are] heavy-laden." From a strictly English perspective, the NASB renders both verbs as passive. This is not an illegitimate translation (the close parallelism may suggest that there is a passive force to κοπιωντες, but I'm not sure about that).
The real question is what we mean when we ask which rendering is most "accurate." If by that we mean "which translation is most literal," then the ESV more clearly reflects the voice of the Greek verbs. If instead we mean "which translation accurately represents the sense of the verse," then I'm not sure there really is any way to say that the ESV or NASB is more "accurate" than the other.