What is the difference between these two TLS suites?
There is no difference. It’s just the way the devs described it; which actually makes sense because X25519 implies ECDH is used.

Also, X25519 is not the curve… it’s the function (to be used with Curve25519). 

Quoting RFC 7748, Elliptic Curves for Security, January 2016, Page 7:


  The "X25519" and "X448" functions perform scalar multiplication on  the Montgomery form of the above curves.  (This is used when implementing Diffie-Hellman.) 


So, to recap: The curves are named Curve25519 and Curve448, while the according functions are called X25519 and X448. And since X25519 implies ECDH, one might regard the additional mentioning of ECDH or ECDHE_ECDSA to be superfluous.