if there are roughly the same total number of monsters available in two different levels

You reasoning is sound, since (for example) you get twice the experience for opponents 6 levels above you. The problem is that there are more low level creatures than high level ones, so it is harder to judge if a few big bonuses killing strong opponents first make up for a lot of small experience drops killing weak opponents later.

Since Lews confirmed an 'insane' amount of resurrecting, I'm back to thinking that this will give you an early boost, but by the end of the game it will probably work out even (or perhaps leave you slightly ahead).
Even if Twoheadedragon's 2 level difference was just due to the order opponents were killed (ie no significant differences in quests done, or things being done more efficiently the second time around), if you are used to defeating much higher level opponents, a couple less levels in the wastelands is relatively insignificant.

In order to 'do the math' on this one, you would need a list of all the opponent types in the game, with their numbers, level and base experience values. The type, number and level info could probably be pulled from the data.000 file of a new game, but I'm not sure if the experience points are stored there as well.
Neglecting quests, you could then calculate what the effect would be of defeating the strongest opponents first, clearing particular areas first, etc.

Of course, when I say 'you' above, what I mean is 'not me'. laugh
I am a little curious about this now, but don't see having the time for it.