Simple interest is not the same as APR. Simple interest is the yearly cost of funds expressed as a percent based on the initial cost of the equipment.

The profit form the equipment should be the determinig factor on whether to purchase equipmentand not the interest rate.

Talk to your CPA as we are not qualified to provide tax advice. Quite often it is more profitable to finance equipment than pay cash. Small businesses are able to utilize tax  code 179 which may let you increase your tax write-offs by finaincing with an Equipment Financing agreement or Lease agreement.