Tap water is better in general (you get free nutrients), but there are some cases where RO is better (or 50/50 is better)
1) Extreme tap water conditions. Where i used to live, the tap water's GH was ~30*, I couldn't keep but the hardiest fish in that
2) Sensitive fish. I found out the hard way that GBR's didn't even like my water if i cut it with 50% RO
3) Sensitive plants.
4) More control over the water's parameters.