Why not do both? Cullen wouldn't cost much to acquire. Get a 3C, get Cullen and keep Sheahan. Both Cullen and RS can play LW. That would also give us more flexibility in who to acquire as a 3C.
Guentzel - Malkin - Rust
Hagelin - 3C - Kessel
Cullen - Sheahan - XXXX
Guentzel - Malkin - Rust
Hagelin - 3C - Kessel
Cullen - Sheahan - XXXX