Contax T and T3 are quite different cameras. The original T has optical rangefinding and no motors, while the T3 has motorized film transport and AF. T flash is detachable, but the body is still larger and heavier than the T3 (which is perfectly comfortable in a shirt pocket). The T3 Sonnar 35 has blistering sharpness, contrast and color saturation, while the T Sonnar 38 is moderate. The T shutter response is instantaneous (best of all my cameras), while the T3 hesitates momentarily even in prefocus mode. T3 film loading is no-fuss automated, while the T requires that you dismantle the camera (almost not kidding). T3 consumes batteries like any modern film P&S, while the T coin batteries can go for years. Top shutter speed favors the T3 (1/1200 vs 1/500). There's a good reason to carry both, perhaps keeping daylight film in the T and dedicating the T3 for fast film and flash situations. The T3 has a shutter-dragging flash mode which is highly effective.
Cheers!