for a solo dev UE4 is better quite simply because there’s the marketplace, an increasing amount of help (tutorials and community) and continuous engine development. it’s like having an invisible team indirectly helping with your game
also even if you’re equally good at both, UE4 is quite better in its productivity (BPs are faster for prototyping or even developing, and the faster code iteration time because of hot reload)
with UDK the advantages are a more stable engine (but find an issue and you’re screwed), and more stable tutorials (zero risk of finding tutorials that no longer work with the version you’re using). but that’s about it