I work on the White Manta and Black Manta so I am biased as can be but I do think they are great boats. As Mase says though, the Black Manta is much more luxurious compared to the White Manta. Cabins are all en suite in the BM and most have double beds or better whereas the WM has almost all single bunks in smaller cabins and only the middle deck is ensuite. The dive deck, saloon, eating area and sundeck on the BM are massive compared to the WM too. The crew, food, service, massages, etc on both are at the same level, I'd say.
In Phuket, there are some other boats more luxurious than the BM but I think both these boats provide good value for money. Then again, as noted, I am very biased!