I doubt it makes any difference whether the drives are 24/7 or not. Same for the computer. Constant heat vs. Heating/cooling expansion of electronics. Both will probably cause death before leaving something off 24/7!
I would assume that Time Machine could cause more wear and tear on the head mechanism since it will be moving it every hour whereas the SuperDuper will only run once a day. On the other hand, the total time each drive actually is in use may be very close to the same; SD just works the drive longer once a day, TM work is usually less than a minute but for each hour the Mac is running. Apple and grapefruit?
Off-site storage is fine if you have the space. Once a week means you may lose a weeks worth of work which might be too much for a business. It may mean a longer first backup for the returning drives, also. If you don't have the off-site space, the whole discussion is moot, of course.
I have two 2TB drives. Similar uses as both of you. Except I did partition each one. And every time I do, I wonder why!
Seagate GoFlex FireWire driveThe TM backup is on a 1.64TB partition with ~27 space remaining. It's set to erase old stuff as needed.
The other partition is a Mavericks 'mini boot' partition using barely 15GB out of 355GB. Thus, almost 340GB is wasted!
The SuperDuper drive is a 2TB Western Digital 'Caviar BLACK'SD just ran out of space the other night, so it now has a 1.25TB partition which has 909GB still available.
The rest of the drive is a partition cleverly named "Empty" and has nothing in it. Theoretically, it is a 'scratch' disk. However, even when it had that name, I don't think I ever used it that way!
I'm not sure exactly what you want to do. Another SD backup will be a smaller version of what you have; it will not really have backups of anything you deleted prior to installing it. Same for the TM backup; it's 'history' will only go back to the first backup it makes. The two drives for use by each app will never be the same.
I suppose you might be able to make and keep them identical with Carbon Copy Cloner. That would be simple for the SD backup (I assume you have it scheduled to run every day at the same time). TM could be a little tricky. While it runs every 60 minutes, the clock starts when the Mac starts up. If that is different one day or it has to be restarted/shut down/started up for some reason, a CCC schedule could end up running during the TM backup time; that would make both actions slow and create a lot of work for the internal drive as well as the OS.
I think your best solution is to simply buy one 2TB drive (Hitachi is my favorite brand) and have it shipped to me to replace the old Seagate.
I'll PM my mailing address...