The IIci cache slot supports L2 cache cards and accelerators. The IIsi PDS slot accepts a NuBus adapter, IIsi PDS enhancements (including accelerators), and, most gloriously, adapters that use IIci cache slot cards.
You want and need the IIci cache slot adapter (plus an accelerator), or a IIsi specific accelerator. Any other upgrade will be disappointing.
You want and need the IIci cache slot adapter (plus an accelerator), or a IIsi specific accelerator. Any other upgrade will be disappointing.