one is not better than the other, they're different and therefore used for different purposes.

Kataphraktoi have great defense, but are a bit on the slow side, so I think they're best used defensively, to block and hold flanks or plug holes in the line. For blocking flanks I set them 2 deep on Hold Form and they'll just stand there and keep the side door closed. Put them 3-4 deep and they'll plug any hole in the line, usually holding even if the infantry gives out around them. They make great gens, and can also be good for a simple frontal smash.

Pronoiai are essentially Feudal Knights with a lower charge. They're great for flanking, skirmishing, and can handle most of the Muslim Cav and hold their own with Catholics.

A good Byz army has both (plus some Byz Cav to shoot and some Lancers as light cav). Use them in their appropriate role and they'll do pretty good.

ichi