Also consider going First class, at least one way. Advance First tickets can be good value and can be mixed with standard in the other direction.
Note that Advance can only be used on the particular train booked. As such what you might want to do is book an Advance (First if you like) for the outbound and an Off-Peak (standard class) for the inbound, which gives you the flexibility to take any train you want coming home. The Virgin website lets you do this - the Off-Peak element being calculated on a half round trip basis. You'd still get a seat reservation for a specific train coming home but if you decided on the day you wanted to leave MAN earlier or later you could do so without having to change your ticket - just get on any train.
Also are you going on a weekend? You'll want to keep an eye on the football fixture list as if there is a Man City or especially a Man Utd game on, in particular a lunchtime fixture, then expect the trains to be absolutely packed. You'll definitely want a seat reservation in that case.