Hi @Syed_Farhan_Ali the MA-PWR-30W-UK is the AC/DC adapter (the "wall wart") to convert wall plug AC electrical power to the DC power barrel that plugs into the separate DC power input on the AP. The MA-INJ4-UK is the Ethernet power injector, so if there is no PoE switch to power the AP directly from the switch port, the "INJ" accessory is the inline power over Ethernet injector.
Most of the time, the AC/DC adapter is used either in labs or to prime APs if needed prior to deployment and sometimes in mesh scenarios when there is AC receptacle within a few feet (the DC power cord on the AC/DC adapter is only about 4 feet (1.2m).
So it's not that one is better than the other, they are for 2 different use cases. Hopefully, there is a PoE+ switch powering all the APs and neither of these accessories would be needed at all 😀
If this MR33 is just sitting on your desk, perhaps at home, and you don't have a PoE switch to power it up, just go with the MA-PWR-30W-UK since it's less expensive than the power injector. And that's assuming the "UK" is correct for what region you're in, there are also -US, -EU, -AU models with different plug types.