BlueAngel- I'm afraid there's no definitive answer for this one; YMMV as different people have different experiences.
It's not that either wash media will, in-and-of itself be abrasive, but rather how the abrasive *dirt* will behave when you wash. With either media, if you press the dirt against the paint and then *move* said dirt while it's in contact with the paint, you'll get marring.
Some people do great with sponges. I'd guess that more people do well with sheepskin mitts but that *is* just a guess.
I myself have to do my initial passes with a Boar's Hair Brush ("BHB") and then follow up with a mitt. And I *have* to use a foamgun in conjunction with both or I'm liable get marring.
If you use a sponge, make sure it's really *saturated* with shampoo mix and rinse/redunk it as soon as it's not. Rinse/redunk very frequently. You want the shampoo mix to flood the dirt away as its dislodged. Don't apply pressure, let minimal contact with the sponge dislodge the dirt so it can be flooded away. Move the sponge in short, interrupted jiggling motions, not long, wide swipes (short scratches don't show as readily as long ones).
Wild guess: you'll do best with a mitt. Fill the mitt with shampoo solution so it seems out while you're washing (the flooding idea again). Move the mitt in short, interrupted jiggling motions, not long, wide swipes. Don't apply pressure.
To be honest, I simply dunno how anybody washes marring-free without a foamgun, I simply can't do it (at least not if the vehicle is really dirty) and goodness knows I've tried most everything over the last 30-some years. No matter how careful I am, I end up having to polish out marring anyhow, but without the foamgun I'd have to do it at least a few times every year.
