For gaming, I assume?
The i3-6100 would pair well enough with stronger GPUs, so I'd go the GPU route.
BUT, the RX550 is not really an upgrade. It's very much in the same general ballpark as a 750TI and, in most cases, would actually be slightly worse.
Even a 560 would be tough to justify as much of an "upgrade".
To me, I'd look to a 1050TI or RX570 from a 750TI -- with a 1050TI probably being a better pairing.