my toons will gernerate mana at the same rate wheter playing solo or in a group of five - so I dont know how you work that statement out. (the MP5 doesn't change according to the number of party members)

the only difference being that a group of 5 will kill mobs using less mana than the same kill if done solo.

mmcookies has the answer in his first post - 2 toons better xp, but slower kill due to lower dps. as for faster, it depends if you are questing or grinding. because most of the multiboxers here have used RAF, and at later levels Questing with triple XPis a better gain than pure grinding (4-5 man teams - mob kill xp is quite low, but your main source of XP would be from quest turn ins)