TPA3251 vs. TPA3255 THD according to datasheet
Comparing the THD data (according to datasheet) of TPA3251 and TPA3255 shows that the TPA3255 provides same/better values into 8 Ohms than the TPA3251 into 4 Ohms. If comparing with a 4 ohm load, THD values for up to 5W output power are slightly higher for the TPA3255 compared to TPA3251.
Only the TPA3251 at 8 ohms has a lower THD (information according to data sheet)