Nine-arm modular multilevel converter(9A-MMC) is a compact modular multilevel converter(MMC) with two sets of three-phase output terminals, and it can be regarded as the integration of two conventional six-arm modular multilevel converters(6A-MMC) with double function middle arms. Compared with two 6A-MMCs, the required submodules(SMs) in 9A-MMC can be reduced, however, there will be some performance tradeoffs, and the performance tradeoffs of 9A-MMC have not yet been investigated in detail. Hence, this paper gives the comprehensive comparisons between 6A-MMC and 9A-MMC, including the required SMs number, the current stress of the semiconductor device, the maximum capacitor voltage ripple, and the power losses of the whole system. This paper can be a guide to make the best use of the advantages and avoid the disadvantages of 9A-MMC. Finally, the simulation results verify the mathematical analysis of the comparisons.