Well, I've already played FF I, V, VIII and saw bits of FF II and FF VI, so I already knew how the system changes from game to game and what elements prevail in the series.
I'd say that it's not true that FF has more battle variety, at least it depends on which game from the series - no. I and the DS Chronicle series sure don't, but V in terms of the job abilites could have, it depend how would you set the criteria (as there are many "title" changes in GS, especially in no. 2 with the item changes and more Djinnies).
But the thing is, everything gets boring after time unless you have to think carefully in every battle - and this happens in every FF, because unless you underlevel you party or in case you set it up really badly and therefore have to be extra careful, after some time you'll find yourself doing the same things in battle over and over again, and sometimes changing the strategy to adjust to a new area/bosses, in case both of them are immune to some of your attacks or have abilities like sleep, instant death and so forth.
So yeah, FF series do have battle variety, both meant that the series have changing battle system and that many of the titles have engaging battle s.(no. I didn't really >.>), however like I said you still find out your optimal strategies and then stick to them. With GS it's almost the same, like you say, the only difference is that because I *think* (I'm not sure) that stat growth is not based upon your title, so you can change your whole party immediately by changing the Djinnies or by equpping a special item and completely change your strategy, whereas in FF you usually have to either reset the game or grind a lot. So in that respect I think that GS has a lot of customization, however unless you like playing with this and constantly twinking your party, you still stick to several strategies like with FF, as I don't remember that I had to often change my strategies, except for the bosses, where you had to think - but maybe that was because I was a bit overleved and normal people would have to consider elemental resistances/weaknessess and so forth.
I guess you probably think that FF is not like this, because a) you didn't notice that it's basically the same, meaning you find your favourite strategies and stick to them in both games b)you really played with the battle system, tweaking the chars and so forth in FF, but like you said not in FF ("it gets boring with the same party over and over..."), so maybe if you tried to change and tweak your party in GS, it would get much more intersting. Like starting to cast spells and not just use your Djinnies and Summons (like I did, I didn't cast much), and especially trying to change roles, like changing Mia from a healer to a damage-basher, or making Garet the healer and so forth. It's that it has a lot of customization, it's only up to you to use it or not, and like in other FFs it can get boring if you don't enjoy so much the changing background and various enemies and seeing the attacks like I did - and not every FF has such a variety of enemies and backgrounds, some just have a stash of rehashed old ones. It's what you make of the game of that creates the experience, both in FF (except no. 1) and the GS.
Ah, now I'm itching to play it again and try this out, but I've got other games to finish.
(when I remember how it was possible in FF V to OHKO almost everything once you got the Ninja class and taught it to everyone, and you could go just with Ninjas having Heal/Summon abilities set. Fortunately I didn't, as I wanted some more creativity, it was only late in the game that I got everyone the dual-wield and then it was soooo easy. Well, it was easy even before, but I felt it was more creative.
)
EDIT: And I nearly forgot - I don't think that GS:TLA was worse, it was as awesome as GS, but I enjoyed the first one more. People who played both games probably recall how the games differed in terms of "feeling" to them, and...there was just something about GS that I really liked, that changed in GS:TLA, one of them being the party which I liked more, and the other the whole feeling of the world.