I like the idea of a 4-3-1-2 if we have Henderson as part of the midfield 3 as I think he's the only one we have with the legs to play the box-to-box role. Also allows Phil to stay as close as possible to Sturridge and Suarez and gives him good freedom.
However, 3-4-1-2 is also the best system for right now as our CB's are a big strength at the moment and against more direct teams, it's a the better tactic to use or teams that use wingers as a basis of attack. So for example, against say a Man City, Chelsea or teams of that mould who look to attack more through the centre rather than out wide, a 4-3-1-2 would work well as it's give better protection against the abundance of attacking midfielders they use and add more to the midfield. However, against your Aston Villa's, Southampton's, Swansea's etc who have a big threat from out wide or from the long ball over the top, 3 central defenders are very useful and there's less emphasis on a need for a midfield, so 3-4-1-2 is a more useful formation. I'd like to think that in time as a team we'll have the personel to go horses for courses and switch between the two systems depending on who we're playing.
However, I've never ever been a fan of an diamond midfield in a 4-4-2. It's just way too easy for certain players to get isolated. Seen it time and again when we tried it a couple of seasons back at Mansfield against much lesser teams.