Two commercially available detector arrays were compared for their use in the quality assurance of patient-specific IMRT treatment plans: one a diode-based array (MapCHECK) and the other an ion chamber-based array (MatriXX). The dependence of the response of detectors on the field size, dose rate, and radiation energy were measured and compared with reference measurements using a Farmer-type ionization chamber. The linearity of the detector response, short-term and long-term reproducibility, statistical uncertainty as a function of delivered dose, and the validity of the array calibration were also examined to understand the stability and uncertainty of the systems. No field size or SSD dependence were observed within the range of the field sizes and SSDs used in the study at both 6 MV and 18 MV photon energies. Both detector arrays showed negligible errors (< 1%) when measuring doses of more than ~8 cGy, but exhibited errors of ~3% when measuring doses on the order of 1 cGy. While the MapCHECK showed a stable short-term reproducibility to within the measurement errors, the MatriXX showed a slow but continuously increase in reading during the one-hour period (about 0.8%). The MapCHECK also showed a slightly better array sensitivity correction with all the detectors having less than 1% discrepancy and more than 90% of the detectors within 0.5% variation, whereas about 60% of the MatriXX detectors showed a less than 0.5% variation and approximately 8% exhibited a larger than 1% discrepancy. MatriXX detectors also displayed a volume-averaging effect consistent with its detector size of approximately 4.5 mm in diameter. Excellent passing rates were obtained for both detector arrays when compared with the planar dose distributions from the treatment planning system for three 6 MV IMRT fields and three 18 MV IMRT fields after the volume-averaging effect of the MatriXX was taken into account.