Microwave breast tumor detection relies on the dielectric contrast between healthy and malignant tissue, and the antenna bandwidth affects the cost, complexity and sensitivity of the system. This paper compares an ultra-wideband (UWB) and a narrowband (NB) circular patch antenna that share the same FR4 substrate, 30 × 30 mm² footprint and radiator but differ in their ground plane and feed. Both antennas were fabricated and measured; the simulated UWB antenna covers 3.56–14.73 GHz, and the NB antenna resonates at 7.79 GHz in its TM21 mode; a finite-difference time-domain (FDTD) model with the measured patch dimensions explains the measured NB resonance shift. Sensing is evaluated with finite-element and FDTD solvers, with each antenna 10 mm from a homogeneous healthy or malignant phantom. Both solvers predict the same 140 MHz tumor-induced resonance shift for the NB antenna and a similar peak reflection-coefficient contrast for the two antennas (19.2 and 21.9 dB in the finite-element model; 7.1 and 9.7 dB in the FDTD model), while the UWB antenna is sensitive over a three to seven times wider spectrum and directs 5.5–6 dB more gain toward the phantom. The NB antenna therefore suits low-cost single-frequency detection, whereas the UWB antenna suits imaging systems that exploit frequency diversity.