The 750 is every so slightly the better card, slightly more powerful (but it’s very VERY close) and that 4 GB would help ever so slightly in certain settings. The 550 is slightly less electrically hungry, but they are both rather easy on the psu anyways. Both cards are very close at the end of the day, the 750 is a little older but they are very close to at par. So if there is a big price difference the cheaper one would probably be the better choice.
However, both are considered as “Entry” level cards, so don’t expect to be doing crazy 4k maxed out settings on either of them, but you should be able to get a solid playable frame rate 1080p with low or medium setting with plenty of games, 720p 60+fps with decent settings in most modern game, and you should get some pretty nice settings and resolutions with DMC5, it’s surprisingly easy on GPUs with some beautiful graphics, not sure about Genshin Impact, never played it.
If you are infact talking about a GTX 750TI then the TI is clearly the winner.