I have two of those 1TB Caviar Blacks. They are great drives. But if the reviews on the product page for the 2TB Green seem to confirm speeds almost exactly as I see on the 1TB Black drives. Obviously the seek time will be higher and the data transfer for small files will be slower. But most of what I do with a drive involves larger files. I guess it all depends on what you intend to do and how much you can spend.
As for the MTBF, I have had a dozen or more hard drives over the last decade. All of them were working just fine when I gave them away because they were too small for what I needed at the time.
As for fragmentation, it happens. But it is easily resolved by keeping a tenth to a fifth of the drive free and defragmenting it every once in a while. Just set it to defrag and let it go while you sleep. For those who may need it, here is a nice freeware degragmentation program.
I quit. I am not coming back.