The C24G1 or CQ27G2U/BK are 144Hz displays, so 120fps is not a problem, just set it correctly on the display and in the Xbox/Game settings.

The C24 has 1080p resolution and the CQ27 has 1440p resolution. On the CQ27 the game looks better, but your Xbox has to "process" more pixels, so you may not have as many fps as on the C24 1080p.

I briefly skimmed some reviews. I think apart from the size and resolution, the two displays don't differ much (due to the price range). It depends on where you play (office desk, etc.) and whether you prefer more fps or a nicer picture.

I think someone else can give you a more detailed analysis, but hopefully I could help you a bit.