Your easiest commute would be from the Mountain Creek area. Probably the longest commute (if you're thinking of living in the Chattanooga area) would be from Ooltewah & East Brainerd thanks to coming through the ridge-cut on I-24. It will vary greatly depending on where exactly you are talking about, but I'd allow 40 - 45 minutes or so if I lived in the county section of East Brainerd or northern Ooltewah.
If you live on Signal Mountain, even though you are close, at times the single-lane road down the mountain can back up during rush hour. One great option would be Hixson/or even Soddy Daisy as you can typically jump on 27 or Hixson Pike & usually make pretty good time.
Hope this helps.