Even at million hour MTBF that only tells us that on average all the drives are dead at that million hours. It doesn't tell us when your drive will die (it only tells us exactly what I wrote in the first sentence.)
So the mantra is "backup, backup, backup..."
Repeat till you can't stand it.
I am doing photo work with Photoshop on my Dell GX400 system with 768 MB ram. I have a 120 GB primary drive and a 160 GB Ext HDD for archiving. The system is dual internal HDD capable and I want to add the 2nd drive. NewEgg lists two 250 GB Western Digital ATA 100 units each for $75. One is rated with a million hour MTBF for use in high demand applications and other appears to be more consumer oriented.
All things being equal, is there any reason I should not choose the higher rated unit. My system does not run 24/7.