They are two seperate designs for a fuel rail/injector.
SARD 800cc side feed injectors
Wolfkatz Side feed fuel rail
Delphi Top feed injector
Wolfkatz Top feed fuel rail
The benefit of sidefeed injectors is that they are PnP with the stock fuel system and wiring. The top feeds require separate wiring leads for installation. In general, sidefeeds are more expensive than top feeds, and they can only support so large of an injector. I believe the RX7 secondaries/SARD 850cc high imp. injectors are the highest you can go (and either will need collar adapters to properly fit them in the rail). As opposed to top feeds where you can go 1600cc's if you'd like