This comparison at The-Digital-Picture between the 5D at ISO 3200 and the Rebel T5i/700D at ISO 1600 looks fairly even, especially if one considers the difference in scale. (I chose the T5i because the T6i samples are even larger.)

The 5D and 5D Mark II look very close at the same ISO 3200 and so any comparisons you might find between the more recent Rebels/xx0D series and the 5D Mark II should be fairly helpful.

Things you should also consider:

• ISO 1600 is the maximum on sensor ISO amplification setting for the 5D. The "H" setting is equivalent to ISO 3200 by doubling the values obtained from the sensor amplified at ISO 1600. That's as far as ISO goes with the 5D.
• The 5D's AF system is 9 points (plus six invisible assist points for the center point when in AI Servo AF mode - again, the 5D mark II is very similar). The AF systems of the T3i/T4i are similar, but the more recent Rebels' AF systems are an upgrade.
• The 5D has no Live View or video capability. None. Zilch. Nada.
• The 5D has no built-in automatic sensor cleaning. Plan on cleaning it much more often with a physical method such as a wet swab.
• Used 5D Mark II bodies are also getting pretty cheap. The increased ISO options, self cleaning, and Live View/video capability might be worth the difference.