(First, I'm not sure if Ishgardian sword-wielders are technically GLA or PLD. Let's go with PLD.)
You're definitely going to see more religious fanaticism among Ishgardian Knights than you would any other type of Paladin. They tend to be very single-minded, obsessed with dragons to the point of ignoring every other potential threat, so I'd imagine most of their combat techniques would be geared towards that. They might have a combat style more focused on jumps, upward thrusts, etc.
Depending on the individual, they might also place less focus on defensive strategies - killing a dragon is a feat that gets you personal recognition from the archbishop, and dying in battle against the Dravanians is the highest honour. They're indoctrinated to have less fear of death.
Ishgardians' focus would be on protecting their city as a whole, not their individual allies. There seems to be less connection between individuals in Ishgard; personal ties are cut very, very easily. So I'd imagine Ishgardian Knights feel less of a compulsion to protect their allies. (This is also highly subject to individual personality, though.)
Overall, I'm guessing the Ishgardians would use a more offensive style of combat.
If you're playing a character who is an ex-Ishgardian though, chances are they aren't as obsessed with "The Crusade" as their fellows are. They'd take a different outlook on life and combat.