To me it seems they both have maskblend applied
They might. It depends on how the Image is loaded (AFAIR).The advantage (or rather one of them) of using alphablended images over masked ones, is the anti-aliased edges can be made partially transparent, so they will blend correctly with the actual background in the game, rather than (say) the background in your paint program.
There are a few real alpha images included in the samples directory of BlitzMAX.