The WiiM Pro Plus and the SMSL SU-1 are both equipped with the same DAC chip, and should sound almost the same (barring any differences in hardware implementation).
I own a WiiM Pro (the version before the plus), which uses a different DAC chip, and although I found it satisfactory, I later combined it with an SMSL SU-1 and couldn't be happier with the outcome.
If I had purchased a WiiM Pro Plus, I probably would not bother using it with an SMSL SU-1, but there may be people here who have done that and noticed a benefit. Maybe they'll chime in.