Both forms are grammatically correct.
In this example, I slightly prefer the second — it flows a little better, to my ear; and pragmatically, people who believe that split infinitives are Wrong might hold the first form against you.
The second form doesn’t seem ambiguous to me in this example, since I pretend not isn’t an idiomatic way to negate I pretend in modern English. There are analogous examples where it certainly would be ambiguous, though; for instance:
My intention in writing this is not to offend grammarians.
Here, not could be negating either was or to offend, giving two quite different meanings: perhaps I have some quite unrelated intention, or perhaps I’m specifically trying to be inoffensive. So in this case, to give the latter meaning, I should rather use the first form:
My intention in writing this is to not offend grammarians.
Language Log has a couple of similar examples.