It probably doesn't matter all that much. They're not causally related to each other in a particularly important way. It's not like you're nailing perfect chronological order anyway if you're playing them whole games at a time.

Chronology isn't particularly important anyway, even where there's strong causal links happening. The real danger is narrative symbolisms and such. For instance, I would generally recommend playing Halo 1 before Reach, as Reach does some stuff in its final act that arguably has a poor effect on how Halo 1 reveals stuff all up through the opening of Two Betrayals.

: Also, have played through both campaigns dozens of times, I've completely
: forgotten the learning curve on each. Suggestions?

Halo 3 varies tremendously by difficulty level. Easy and normal are both cake, while legendary actually has damage balancing not terribly different from Halo 2's (although it's easier than Halo 2 for a number of reasons, like brutes being slow and bad).

ODST's lower difficulties feel harder than Halo 3's, but the scale is more compressed. One thing to remember is that even on normal or heroic, and even if you're reasonably good, you're likely still going to lose a bit of non-regen health occassionally since the shields are so weak. So you can still feel challenged in some sense or another to play better on fairly low difficulties.

If she's not particularly good, normal or heroic would probably both work for Halo 3, I'd lean toward normal for ODST.