This may not make any practical difference, but if I remember my high school physics, a mirror is going to reflect visible, IR, and UV light to the same focal point, as long as it's a front surface mirror.

A lens, on the other hand, will have a different focal point for different wavelengths of light, because the refraction is a little different - chromatic aberration to the photography geeks. A glass lens is also going to block short wave UV and probably long wave IR. So the metal mirror should give more light in a smaller area, and thus more heat.

A flat fresnel lens is a lot handier than a small parabolic mirror, though.