if u have time to prepare all supporting documents, go for 1st round. if u dun have enough time, e.g. need retake G or TOEFL, go retake and apply the 2nd round or 3rd round. Dun rush for the first round while compromising your competitiveness. Generally, 1st round the chance to get selected is higher if u r really good as school need CONFIRM ENOUGH good candidates as early as possible, so most time, quota for 1st round is relative flexible, comparing with later rounds.