Imo the last series that was good at keeping the original music and dub-only tracks was BW - BW2. Of course I prefer the original BGM kept over the dub tracks but even then they were at least tolerable compared to the dub music done in XY and SM (XY more so) imo. In terms of VAs, BW was where Sarah Natochenny started to improve and now in SM, she has shown how far she has come since her first days voicing Ash! I also like the voices of Cilan, Stephan, Trip, and some others too. I can't find any particular dub voice I like in XY however. Alan in particular was so emotionless lol. I guess Clemont was decent but that's all that I can think of. : /
SM is a step in the right direction though imo. Music wise they're hardly better but voice acting wise, there's dub voices that I actually like again. I personally like Kiawe, Lana, Professor Kukui, and Rotom Dex are my favorites while Mallow, Lillie, and Sophocles are OK. Samson Oak is the only one that I don't like from the cast and Team Rocket...will they ever have good voices again? Jessie and Meowth in particular.
In the end, I suppose I prefer both but I watch the Japanese first since I can see the episodes quicker rather than waiting until the dub comes.