Final Fantasy XI is my favourite FF title. period.
It has a fantastic story, and IMO a great player base of fun people (or did at the time i left in 2010)
The world has excellent art design that is worthy of the franchise across a world so gigantic that it dwarfs the size of any other FF title. The drawback in the day being that progressing to the very top of the power curve takes a long time. Much longer than FFXIV anyway. But the game is epic and has that play it all night charm.
Get it before its gone.

That written, i believe FFXIV has the higher 'potential ceiling', and have from the start, but the real date of note for this game is September. Playing it before then is really just preparation for what is to come and some preview content to enjoy as 2.0 begins to land.

My suggestion is, if you really are interested in FFXI, it is a masterpiece and you'd best get it now before its gone, and before FFXIV reaches 2.0.