Hi Mike,
I had 5T's/5C for about 8 months and then upgraded to 6T's/6C. My listening area is approx 20ft wide by 12ft deep with an opening on one side to the dining area (additional 10ft wide) so my area is a medium size room. I think the differences will be more significant in a larger room but I'll give you my impressions. I loved the 5T's and thought they gave me a great depth with solid mids. I had them crossed at 80 to my 8D. With the 6T's the mids are considerably fuller and the low end is much stronger. I've since changed my towers to cross at 50/60 to the 8D although the 6T's can be used full and also sound great. I use a second receiver (old pioneer sx-1050) and comparing the 5T and 6T with that receiver is more striking since there is no sub in that setup. The 6T's are considerably stronger in the mids and lows but that's to be expected from the specs.
In all I think the 5T's are a fantastic speaker especially if you use a sub. If you listen to music without a sub the 6T's are a better bet. I'm happy I did the upgrade. The only disappointing aspect is listening on my onkyo 805 to music on all channel sound and hearing the 4B's try to compete with the 6T's! Also keep in mind that the boxes for the 6T's are big enough for you to pack another person in. Make sure you plan on somewhere to store them!
In any case I think the difference from the 5T to the 6T is more like night and day for 2 channel music. The difference between the 5C and 6C is more dependent on the size of the room and how far you sit from the TV.
Hope that helps
Jason
Speakers: Aperion 6T's, 6C, 4B's, and 8D
Receivers: Onkyo 805 and Pioneer SX-1050
DVD player: Oppo 981HD
TT: Pioneer PL-71
Amp Selector: Niles DPS-1