Hi there.
I have travelled to the MEN arena many times on the metro (tram). It is a really smooth journey and very easy. There are several metrolink stations where you park up, buy a ticket from the machine and that's it!
So for example I often go from Prestwich or Bury, park up, buy the ticket and go straight to Victoria which is the station right next to the arena, literally under the same roof! Just hop off and climb some steps (or use lift)and you are in the 'City room' of the arena.
Catching the metro back is just as easy. Just do the journey in reverse.
The only thing you need to check is what time the last tram runs, and if it is late enough for you. They run past midnight I think but not all night. Sometimes the trams are quite packed but everyone still crams on and it's still the best way to go and quickest too. If you have ever been on the Tube in London then it's very much the same but overground.
Prices are really reasonable, much less than going by train and so much easier. There is about a 10/15 minute walk from Piccadilly to the MEN.
A full map of the Metrolink stations, and times is on the link below:
Metrolink Home Page
If you really wanted to go by train, I always use Virgintrains. It's easy to use and they have the best deals usually. The earlier you book, the cheaper you can get the tickets.
Try different combinations like return, 2 singles, return to a different station (you get off earlier) and even look at the first class if they do it for that journey. I have often got 1st class to London cheaper than standard. It all depends on when you are travelling.
Also if you are student or concession or have a family it may be worth buying a £20 rail card and then getting a discount and the kids all for £1. I have also saved money doing this to London in peak time. The only thing is that at least one of the kids has to be with you.
Hope this is of some use. If it comes to buses however....I know nothing..!