I had an 18-1/2ft TT and have a 21ft class C. Here are my observations in regards to space:
With the class C, I have the dedicated cabover bed. That makes up for the lost space due to the cab and engine. Without a cabover bed, space would be much tighter.
With the TT, I did not have a dedicated bed. I had a convertible dinette and sofa. Some of the 3ft less in overall length of the box was added back with the tongue and hitch. With a 21ft box, I would have gained 3 ft of inside space, but the total length would have increased due to the tongue. Spacewise, I don't think I could have had a dedicated bed, dinette, and sofa even with the additional 3ft.
With the TT, you need to add the length of the tow vehicle. With the class C, you can choose not to tow a toad so your length remains at the rig length. If you choose to tow a toad, your total length increases.
I recommend you look at floorplans and "daydream" about how you would function in them. Whether you find one or the other better will be a personal choice.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)