Well fort a start...cost but they are both considered low dispersion or Extra Low Dispersion Glass (ED Glass).
FCD-1 (sometimes called FPL-51) is a lot cheaper than FPL-53 and I think easier to cast blanks, especially large ones.
FPL-53 is probably considered better in chroma correction but FCD-1 is not bad. Actually ....I should say FPL-53 bends light differently compared to FPL-51.
Consider most APOs have one element of ED glass the other elements are another glass. Although from what I have read, in the case of this Skywatcher, they talk of more than one element being FPL-53 (ED glass). This is probably due to the Petzval type design so it should have very good colour correction.
As for which is better....probably the
9 pages on CN is best to read....boils down to what suits your wallet (edit: and the abilities of the designer and quality of manufacture).