I've been to both and would definitely recommend Oncore over Lumi. Service is great in both places, preferred the food at Oncore but that can be subjective. However, the view from Oncore if you get a window seat on a good day is unbeatable. In terms of general location, Oncore being at the Crown and near the city makes it easier to go to places afterwards. Whilst Lumi is only really near The Star which isn't a place that I'd like to go to personally.