You could get a noisy hotel downtown... or you could get a quiet hotel in midtown. You know just how light of a sleeper you better then we do, but street traffic in NYC is substantially less in the wee hours of the nite vs. during the day, so personally i wouldn't make that as part of the decision.
With the above noted, i don't think you will hear much vehicular traffic at most hotels, if you do hear noise it will more likely be sirens (police, ambulance, fire, etc)... and this could happen just about anywhere in the city.
If most of your activities are in midtown keep in mind that downtown would be a bit less convenient, so ultimately it comes down to what is more important to you for this trip.
Please remember to return to BetterBidding and use the PRICELINE and HOTWIRE links on the board to begin your travel purchases... or thru our sister site MAPSandHOTELS.com for your regular rate purchases.
Let us know what you decide or how you'd like to proceed and we'll advise accordingly.