I played FFXI about 5 years ago, and loved the **** out of it. I had stopped, and started it up recently, only to find all my friends were gone, and the servers were practically empty compared to where they were, also there were no low level peeps, so leveling up a new character is not as fun anymore...
All XI servers have large playerbases still, there are just none in the starter cities and all low level alliances take place in different places to where they used to. Gusgen mines (14+), crawlers nest (30+) etc are the big leveling areas now.
Currently over nearly 2.5k people on my server on FFXI (and they are all around that during peak time). FFXIV servers have a tiny fraction of as many players, it's as dead as any MMO can get.
FFXIV is honestly nothing like XI at all anyway, enjoying FFXI will give you no indication of whether you will like XIV anymore than Aion. I'm a pretty hardcore XI player and I find very little enjoyable about XIV right now. Only thing they share in honesty is the name and a few designs copied over, nothing else is the same.