They're pretty different beasts (despite both being strategy RPGs). FFT has a good, serious story. Disgaea is good for completely different reasons. The story is just silly, but it's genuinely funny and has endearing characters.
In terms of gameplay I honestly prefer Disgaea (and no, not because of the optional mega grinding, I never do that crap and you don't actually have to). Admittedly it's far from a perfect system, but mechanically FFT is a broken mess. Yes, it pioneered the SRPG genre and is important historically, but that doesn't change the fact that it's a broken mess. There are so many ways to completely abuse the system and the game has some major difficulty spikes that will punish you if you're not abusing things.
So I guess I'm going to have to disagree with the majority here and say it depends on what kind of mood you're in, but I would honestly lean towards Disgaea.
(Also don't touch FFT unless it's the PSP version because the story is the major appeal of the game and the original translation was abysmal. Actually I think there's a smartphone/tablet version of the game that also uses the PSP translation, but I haven't played it so I can't comment.)
Don't worry Kev, you my friend are not alone.... while my arguments may be weak, and I can objectively see in many senses where Tactics is the superior title.... my vote still goes for Disgaea.... However, my reasons for this are mainly subjective (Disgaea caters to my personal taste way more...) and as such I haven't got much of a leg to stand on here....
The main appeal of Disgaea over Tactics to me is the vast amount of content and control over your team. It has been ages since I played tactics mind you, but I don't recall having a dire need to create and beef up custom troops in Tactics.... In fact if I recall correctly, I found myself hard pressed to have little reason to even worry about custom troops by the end of the title..... On the other hand story characters felt completely essential to victory, save a few of the custom classes that were just ungodly powerful by virtue of their mechanic or skill. Disgaea is the completely opposite end of the spectrum in this regard..... you can virtually make any class a force to be reckoned with and if you are at all concerned with post game content and optional stuff you'd best do just that.
Now, I'm no grinder by any means.... I like to play a game, enjoy the content and move on for the most part..... I usually could care less about maxing out this or that.... However with the Disgaea titles, becoming super-leveled always had a pay off that proved worth it to me.... first of all, I always found the item worlds to be addictive as all hell (I am certain I logged more hours in the item world of each disgaea title vs what I logged totally with all of tactics).... secondly it wasn't just large impressive numbers I was after.... those large impressive numbers enabled me to take on new maps and see new content....... Those two factors alone kept me on-board for more disgaea well beyond the credits which is something I rarely find myself doing.... usually when I finish a game, I am eager to move on to the next....
Lastly, disgaea pioneered the over-the-top niche anime styled RPG we've all come to know (and hate-Lol).... I just happen to eat this style up.... for as re-hashed and recycled as it has become in the present gen, at the time of disgaea this was a VERY original flavor to savor.