First, yes Massive MIMO Can be used in two modes FDD and TDD.
For TDD, Downlink beamforming in Massive MIMO relay on uplink pilot measurements—exploiting reciprocity and TDD operation.
For FDD, Massive MIMO uses a predetermined grid of beams with UEs reporting their preferred beams, mostly in FDD operation.
The original Massive MIMO concept assumes TDD and exploits reciprocity for the acquisition of channel state information (CSI) at the BS.
For the question, which is better?
TDD is better in performance due to TDD is using the same frequency for downlink and uplink and as different frequencies have different channel effects, So TDD is better in channel estimation as compared to FDD ( as FDD utilizes different frequencies for each transmission direction).
So TDD requires less channel estimation time, hence has system has more time for data transmission, therefore, increase in system spectral efficiency.