Starter set is totally worth it.
But I think this has been said before. Every army is good. Every army is up to date with the new edition and newest models. Pick the faction whose models interest you the most in terms of art style or for painting or for background and you can find a play style that makes sense for you in that faction.
That being said, Legion is known for having very strong individual beast models and tends to play a more aggressive, first-strike style of game with very few infantry (relatively). You can get away with having as few as 6-7 models on the table for Legion at tournament points levels (35 or 50), they'll just mostly be heavy beasts. You can of course, go the other way, and play them as a heavy infantry army, you'll just have to pick a different caster to make them work.
As a rule, WM/H are much less susceptible to net-lists and flavor-of-the-month power-creep (since everybody creeps at the same rate), so you're better served by picking a faction that appeals to you for any reason OTHER than perceived battlefield strength. Winning or not will be much more a function of how you play than what you play.