TRIM does not affect the reliability at all, though obviously it is an extremely useful feature to have.
It's possible that each of your two unreliable drives are so unreliable that you'd be better off with a single more reliable drive, though this is unlikely. Let's assume that your reliable drive has a chance of failure of 1% over the course of a year, while your odds of failure of the less reliable drives are 20%. Further, let's assume that the two less reliable drives would fail in a statistically uncorrelated manner. And once one drive fails, you will replace it and rebuild your array within 24 hours.
Your odds of failure using the more reliable drive are then 1%. Your odds of losing at least one of the less reliable drives are approximately 36%. If you lose a single drive, though, you don't care unless the second drive fails within 24 hours. The odds of that happening are approximately 0.00055%. I calculated this by doing 20% / 365. I believe my math is correct, but you'll want to double-check.
In other words, a RAID-1, assuming drive failures are uncorrelated and assuming you replace a failed drive and rebuild the array within 24 hours, is going to be far, far more reliable than a single drive. Of course, the RAID-1 controller itself can fail. Or your computer can be struck by lightning, or your house can go up in flames. Or you can delete a file you didn't mean to. Always back up your data.