I need an ispec-B Shimano shifter and SLX is the only thing available to me today. Is there much difference between XT and SLX?

Not really, the XT ones feel a bit nicer

Only a marginal difference really.

Please define. I’m coming from SRAM X9

In that case, you’re laughing: Shimano works and it feels smoother too.

Not much difference but I’m not sure if you can change two at once down the cassette with SLX can you? You certainly can with XT/XTR but not the SLX ones I tried (albeit they might have been last year’s).

Shimano works and it feels smoother too.

Not SLX imo. It’s clunky and cheap feeling, and not nice-clunky in a SRAM way. XT is better/as good: light action but solid feel.