There are some differences.
1. fire forming. In an SLR you don't. You size new 243 brass in your 6SLR die, load, and shoot. With any AI you have to fire form to blow the shoulder out.
2. case capacity. I've measured my fired 6SLR cases compared to brand new WW 243 cases and the water capacity is the same. In a 243 AI you will gain ~3% in case capacity, in round terms about 1.5-3gr.
3. Velocity. The 6SLR can get the same velocities as the 243, with a long neck and a better shoulder angle. With the 243AI, in real world velocity, you can gain about 50 to 75 FPS for a given pressure. There will be those who may claim 150 FPS, and I'm sure they're getting it, but there is only about 150 FPS between a 308 and a 30-06 with a ~21% increase in case capacity, so if they are getting that much they are running higher pressures.
4. brass. The easiest way to make SLR cases is with WW brass. It's what I run today. If you want to run more expensive brass you're going to have to turn necks because they (Lapua, Peterson, Alpha) have thicker brass in the shoulder that gets pushed into the neck when you make the SLR. Something to consider though, I have read that Alpha or Peterson is making 243 small primer brass. That may be worth turning necks.
I think the SLR is better, you get the advantage of a 30* shoulder and a long neck. The AI gives the case a good shoulder but does nothing for the neck and adds a tad bit of capacity and velocity, but they can both get you north of 3100FPS with 105 class bullets in long barrels.
I'm personally not a fan of AI's made from cartridges based off of the 308 (or pretty much any modern case). They are all already a pretty straight walled to begin with, so it buys you very little for your work. When PO Ackley was blowing out cases like the 30-40 Krag it went from a tapered case with a 308 capacity to a straight walled case with a 30-06, that's a big improvement.