I think Henderson is the main player of our midfield. Imagine where we would be without him. Joe Allen is not the same class, unfortunately.
The way our midfield is set up helps enhance the best attributes of the entire lot of them.
Henderson, Gerrard have both had phenomenal seasons, but I'd say Allen has looked very good too for the most part, as has Coutinho. I think Allen is more handy in possession, has better ball control than Henderson, who is the better presser and better in and around the final third. Henderson's quicker, but Allen can manouvre out of tight positions etc.
Lucas has also had some very good performances but I feel his importance to our midfield setup has declined this year. I do think he'll have a big role to play with European games next season and (hopefully) 50+ game season.
They've all got their positive attributes. I think Allen is a very talented player, and will only get better. The only midfielder who'll be a different class to the rest in a year or two's time is Coutinho, he's the one who can reach world class levels, the likes of which Stevie's performed at the last 3 years and did for most of his prime. That's not to say that Hendo/Allen can't become great players, they can, I just think the only one with a clear difference in talent level out of our lot is Coutinho.
I hope Allen steps in and plays at the level he's been performing at recently. He's a top lad with a great attitude just like Henderson. There's a reason why the squad is so together right now. The guys on the bench are always supporting the guys on the pitch. Positive minded team players are essential for this, which is exactly what Hendo and Allen are.