Ultra Wideband (UWB) radar is a promising emerging technology for breast cancer detection based on the dielectric contrast between normal and tumor tissues at microwave frequencies. One of the most important considerations in developing a UWB imaging system is the configuration of the antenna array. Two specific configurations are currently under investigation, planar and circular. The planar configuration involves placing a conformal array of antennas on the naturally attened breast with the patient lying in the supine position. Conversely, the circular configuration involves the patient lying in the prone position, with the breast surrounded by a circular array of antennas. In order to erectively test the two antenna configurations, two 2D Finite-Direrence Time-Domain (FDTD) models of the breast are created, and are used to simulate backscattered signals generated when the breast is illuminated by UWB pulses. The backscattered signals recorded from each antenna configuration are passed through a UWB beamformer and images of the backscattered energy are created. The performance of each imaging approach is evaluated by both quantitative methods and visual inspection, for a number of test conditions. System performance as a function of number of antennas, variation in tissue properties, and tumor location are examined.