My unscientific view is that the 630 and 721 are very similar in performance, despite the difference in tube count (30 tubes for 630 vs. 21 for 721). I watched my 721TCS for about 4 hours yesterday and was again impressed with its great picture & audio, and its stability.
The 630 does have extra IF stages in the video and audio, which might make it better in weak signal areas, but that's not a factor now, when we are not listening to distant stations. It also has a 6AL5 DC restorer, and two 5U4G LV rectifiers rather than one. That accounts for a few of the extra tubes.
If you compare the rest of the tube lineups, you see that the 721 often uses a single multi-function tube where the 630 uses two. For example, the 721 uses one 12AU7 tube for the 1st and 2nd video amps, where the 630 uses two (6AU6 and 6K6GT).
In that sense, the 721 just seems like a newer design, not an inferior one. Older TVs used more octal tubes for single functions. As time went by, the general trend was to use more glass mini tubes that could do multiple jobs. That not only reduced cost, but it may also have improved reliability, since fewer parts means fewer failure points.
Perhaps others will disagree, but that's my quick take.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html