Biggest advantage is not having to deal with cable stretch on the cable driven.
With todays indexed shifters (not the old friction shifters), once setup - either works great. However over time, esp. with a new cable, there will be some break-in adjustments required on cable shifters.
Biggest improvement to shifting is cog design, and shifters. Derailleur is much less significant in improving shifting performance.
Unfortunately with the SunRace freewheel, there is no upgrade path I'm aware of. That leaves only the shifter. If you already have an XT or XTR shifter, you are at the pinnacle. If you have Sora shifters, then the XT will make a different in effort and feel. Shifters move the derailleur, and better ones have trim built into their indexing. The derailleur is just along for the ride.
The Di2's are really cool. Once dialed in, should be pretty much maintenance free other than batteries and cleaning. Likely wont give you any gain in performance on a properly setup cable mech. However that applies to both. Proper setup.
A disadvantage is with battery - it dies on a ride you have no shifting. Even with mech tuning issues you can fudge it a bit to get a gear change... however mech will not let you down like batteries.
