I don't see how doing two separate teams would make anything go faster over the long run. Your kill rate w/5 will be faster and you have less risk of deaths due to higher-level stuff or tripping on adds, so you are quicker on the move through mobs, instances, etc. The only slowdown would be those gathering quests, which (if you're leveling through quests) will be the same speed as you are gated by things like number of people gathering/killing in the area, number of gathering nodes, whether you have run speed, quest item drop rate, etc.
As an example, I was soloing down in the south barrens for the Lok/Kuz/Nak skull quest and those mobs (by the time I got the quest) were about +4 to +6 to me. If I'd have been a full group, I'd just have mowed through there and finished the quest while it was still red to me. Good xp, loot appropriate or even a little ahead of me, etc.
So, I personally prefer 5 in all cases except perhaps one: when you have a mix of classes that cannot start in the same area or that you would prefer to start in another area for better quest progression.
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