I do plan on buying the full version.
So, when you say my chances are higher, do you mean that if my main drive goes down during an incremental back-up, there is a chance that I will lose all of my past data as well as the new data? So, a drive failure like this would cause some kind of corruption in the total back-up file that would require some advanced recovery method?
When you start talking drive failure, almost anything can happen. If your primary drive fails in a way that corrupts the original file, SuperDuper would probably not copy the corrupted file to the backup drive.
But, honestly, I think that's worrying a bit too much. Let's take it in steps. I'm paranoid about backups. My goal is to reduce the amount of time that an important document or piece of data exists in only one place. To do that, I:
1) have SuperDuper clone my internal drives to my external drives, every night, at midnight. Just doing this will save me 99% of the time, and (sadly) is more backing up than most people do (maybe TM will change that).
2) I also use the online service described above. It is near real-time, but is slow on big files.
3) My point about the flash drive is this: let's say I write a really important document at 2:00 pm. It won't be cloned by SuperDuper until midnight that night, so for ~10 hours, it exists in only one place. There are lots of ways to address this, including just ignoring it and accepting the (very slight) risk that the drive will fail during those 10 hours. If you really want to be paranoid, like I am, you have lots of options, though. You could launch SuperDuper and run a regular backup. You could e-mail the document to yourself at gmail or something, so a 2nd copy exists in your inbox. Or, what I have found is very simple, mentioned above, is I have a shortcut on my desktop to a SuperDuper backup that simply does an incremental backup of the Documents folder to the flash drive, excluding big files, video, etc. Whenever I want to run the super-paranoid-backup-now-to-flash backup, I just double click on that shortcut (oops, I mean alias). It takes 5 seconds to run, and it is done. You need the full version of SuperDuper to do this.
I don't know of any way to trigger SuperDuper automatically, which is why I just use the alias on my desktop.
If you are really into backups, check out Mozy. It is slow, but it maintains a backup offsite, so if my house burns down I still have all of my pictures and stuff. I don't recall how much it costs, but it has a price for unlimited amounts of data which isn't too bad -- I think I have over 500 GB backup up with them at this point. The OS X version is still in beta, but has worked for me so far.
Oh, and once a month I also burn a DVD backup set of the stuff I care about (mainly pictures and video of my 2 year old son) and I just throw that in a drawer at work.