The Ent had only partial impulse power and little if any warp reactor power when Spock said the line. Reliant seemed to be basically fully functional yet.
So it's just all guessing as to what Spock meant, but I doubt either ship had a completely decisive advantage when both were in peak condition. They were from the same era after all and Ent was a beefy heavy cruiser.
The nebula completely saved the day for Kirk and gang IMO. Once Kirk spent the wild card of the prefix codes, I think Ent was done for in open space where Khan could easily unload on crippled Ent.