is keying easier with progressive footage?

or is it not that big of difference

yes.

Oh...the forum want's more than 10 characters in a response!

I've had an easier time keying with 30p than 60i. It was different cameras though. Sony FX1 in HDV (60i), an HVX200 in 30p, and my HMC150 in 30p. Tomatoes, apples and oranges camera-wise but I know how to shoot greenscreen so the footage was of the same caliber lighting. None were difficult but it seems a lot less time needed to perfect the edges. And motion blur is easier to deal with in progressive.

Maybe I'll do a test with my cam shooting both styles same subject....if my clients would leave me alone! HaHa!

is keying easier with progressive footage?

or is it not that big of difference

It is important to key progressive footage, but that is why AE can interpret footage. Just control click>interpret your clip and you should be able to choose upper (HD) or lower (SD) fields, and it will de-interlace right before your eyes -if it is interlaced.
Sometimes it does that automatically, but you can control click>remember interpretation and apply it to all the other clips if you need to.