Quite often, one performs better than the other. Sometimes, the built in works/looks better, but other times, it's the input device (player) doing the better job. YMMV. Some trial and error involved, especially regarding specific content, etc.
Some arguably, higher end/high budget folks opt for a completely separate scaler (outside of the player even).
I was wondering what is the point of getting a 4K Upscaling Blue-Ray player if the the T.V can already upscale 1080p content to near 4K. I'm going to be getting a 4K tv so I just want to know if I should get a that Blue-Ray or keep my old one.