Good question, and one worthy of a lot of serious consideration.
I am not a carver, however, the carvers on this forum don't post very often.
Here are my thoughts:
I can't imagine carving chisels being beyond the range of the smaller T3. The eight inch (to start with) wheel is larger than most dry grinders. The 50 per cent duty motor is probably more than sufficient, unless you have a very busy professional carving sharpening service (doubtful).
The plastic housing is probably plenty rugged for use on a bench where is is not moved frequently.
That said, the T7 comes with the stone grader and TTS 50 diamond dresser standard. You will want both of these, so add about a hundred dollars to the cost of the T3 in your comparisons. The more advanced water trough is more for long knives than carving tools.
My Tormek is a T7. I tend to overbuy, so, if I was buying a unit today, I would probably buy a T7. I would give very serious consideration to the T7.
I hope some of the carvers will chime in.
Ken