the enlarged pontus 15 is essentially the same dac as the venus 2 - the company is just doing some (confusing and stupid) relabeling of the models -- this company is infamous for outsmarting itself with its idiotic, constant model revisions and renaming
all things equal, i2s or aes input would be the best input if you have a streamer with a superior clock than the already quite good built in clock inside the dac (sorry i am not familiar with your smsl unit...)
otherwise use the usb input to allow the dac’s clock to control the festivities
cables matter but keep it reasonable don’t go crazy -- just buy reasonably priced well made cables of a known brand or quality, $100-200 for the cable should be enough