Would I be better off with the commercial version, such as the Comet GP-6NC, of some of these antennas rather than the HAM version ? How much better would these antennas perform over say an inexpensive MFJ-1740 or RS 20-176? How well would a tuned VHF antenna pick up the UHF signals?
The commercial versions would probably be more robust and perform a bit better, but at a much larger price tag than the ham versions. Commercial versions are made to be mounted and pretty much forgotten about for many years under constant and critical use (remember that they are designed to be mounted on commercial towers where you pay for tower space and pay dearly for someone to climb the tower to mount or work on them).
Ham antennas are designed to be self mounted and maintained as well as cost being more important than working forever without being looked at for decades. Many are made using thin fiberglass tubes (or thinner alluminum) while the commercial versions use fairly thick alluminum or fiberglass for their construction. The commercial versions may also be a single length tube while the ham version may come in sections (for reduced shipping costs). While the commercial versions may last longer, you probably can replace the ham version three or more times for what you pay for the similar commercial version.
The two antennas you mention (MFJ-1740 or RS 20-176) are simple ground plane antennas that provide you with no gain. Many of the ones I linked to have 6 or more dB of gain, which should provide better performance.
A tuned VHF antenna will pick up UHF signals fairly well, but not as well as one tuned for UHF (either as a single band UHF antenna or dual band VHF-Hi/UHF antenna).
