Ok so personally i found T50 the best to grind on for a couple of reasons:
1. You get better options in the spins at the end.
2. You can beat them just as fast as the Lvl 3 as the AI doesn't you a human deck very well (no synergy use)
3. You get more points (from what I understand points=electrum gained)
4. You also have the oppurtunity to hit up a rare card made farm (some T50's leave a rare card deck open for us to farm rares off)
Thats my opinion but you would need a decent deck to do it and Life is able to relatively easily if you use the right set.
Btw, Rahlious is right about the deck, a great setup.
Yours
Scyther