If you ask yourself the question is Isaiah Thomas ready to play in the NBA today the answer is no. At 5'8 Isaiah will have a difficult time guarding taller point guards and his quickness would be considered average for his size. There is no distinct advantage he has on the next level. In addition, there is no collective bargaining agreement in place which means there's no guarantee of when and if the NBA season will start. From that perspective he should stay in school.
If you ask yourself the question does Isaiah Thomas have anything left to prove at the collegiate level the answer again is no. Isaiah has shown he can make the transition to the point guard position and few people in the country can match his toughness and shot making ability. It would be nice to get to the Final Four, but not necessary to validate his skills. Without signing with an agent the draft is an opportunity to see where he stands presently and if he doesn't like the results he can always come back. From that perspective he should test the market.
At present Isaiah Thomas is projected at the top of the 2nd round in the draft. That could all change depending on what other underclassmen decide about their eligibility and what scouts think after his NCAA tournament performance. If he came back for his senior season chances are he would be at least a 1st round pick, but there are no guarantees in this world and it never hurts to check your current status. My guess is he'll be back and I wish him luck either way it goes.