As far as classical Latin is concerned, neither.
I went through the 28 occurrences of praeparatio (in any case) in a classical corpus, and I found only one case of "preparation for something" expressed with a preposition:
Cicero, in Tusculanae Disputationes 3.30.9, uses
praeparatio ad + accusative.
It appears to be more common to use genitive.
For example, I would render "preparation for his arrival" as praeparatio adventūs eius.
Also praeparatio ad adventum eius by analogy to Cicero is fine, but I find no support for preaparatio in with either accusative or ablative.
See also the L&S dictionary entry for praeparatio.
However, this answer only applies to classical Latin.
You commented that praeparatio in is indeed found in later authors but I don't know enough of later use to be able to comment on it.
As pointed out by C. M. Weimer in his answer, you can say praeparatio in expectatione alicuius for "preparation in expectation of something".
This is not attested, but it appears to be a reasonable extrapolation from known classical material.
However, to me this is not significantly different from praeparatio ad aliquid, "preparation for something".
As always, the best choice depends on context.
See his answer for a better discussion of this point.