As car use is for work, you can deduct the lease payments. Cannot deduct purchase payments, ie-finance
EDIT: I checked with those more knowledgeable than I (enrolled agent before the IRS). Consult your own tax advisor.
I was told that you can deduct purchases as a combination of interest expense + depreciation expense. There's a limiting factor here in that section 179 + depreciation expense + bonus depreciation can't exceed $11,000 or so per year for the car (and even then some states don't allow bonus depreciation), so you will have to consider a longer depreciation schedule to recoup the purchase -- this could be up to 15 years! So, this would seem to be in favor of leasing when it comes to high-cost cars for business use, but if you desire you can indeed deduct a purchase.
That said, if you intend to drive a lot it may be that you'll get more bang for your buck using the 56.5 cents per mile deduction instead of actual expenses, and if so you can't consider either lease or purchase payments as part of the equation. There are some conditions that would allow you to take interest expenses in addition to the mileage discount, but I was told that at today's interest rates, it may not be worth it to deduct the interest expense. The important thing when you use this method is to keep a log of every trip - source, destination, mileage, reason for trip.
Hope this helps...