Dell S2715H vs LG 29UM67P vs ASUS MX279H?

For me its between the Asus & the LG (fairly certain the panel in the dell & Asus is the same but the Asus has far better connectivity & goid quality speakers)

Your choice is going to be solely based on the choice between 16:9 & ultrawide.
The ultrawides are truly great for desktop use with 3 or 4 windows open , & also great for games that support the resolution properly (driving & fps games benefit massively from the extra wide field of view)

Bear in mind the 29inch lg height wise is about the same as a 24 inch screen but elongated horizontally.