In my day job I'm an engineer for a storage company. I didn't mean to say spend more money on higher priced SSD's. The biggest failing point for mechanical drives is in laptops where they are dropped, which throws usage statistics off for longevity. I test on server farms where we are writing constant I/O streams far more than even a power user would perform, even higher than most Enterprise data base systems would write. We have mechanical drives that have been used well more than 5 years. Enterprise storage systems like EMC, IBM, Hitachi are nearly all mechanical. They are slowly introducing SSD as enterprise solutions and even some of that is MLC depending on how it's used.
Again nothing against SSD's they are the future, and I love them and use them. I think how you use them to get the most out of them.
SSD's do fail and it's not about longevity it's that they fail differently than mechanical drives. When an SSD fails your data is likely gone for good. On a mechanical drive the data is usually recoverable. Where mechanical drives biggest failure point is someone dropping something, a SSD is more likely to be damaged via power spikes, overheating, etc.
Segate's hybrid SSHD is a great technology is combines the best of both and is great for data. You can get the read write speed because the cache on the drive is a large SSD, usually 8 - 32GB NAND. It also has an onboard database to store recently used files on the SSD as a copy, so you work with the files as an SSD and it transfers it back to the HDD. The files are on the spindle and your data will usually be recoverable. Plus the price advantage for SSD is unbeatable.
For an OS drive MLC is a no brainer, price for performance. I would not recommend MLC SSD for long term data storage, unless you are backing up to mechanical or cloud.