I purchased the 512g version, and when I heard about the 1tb model having 6g of RAM, I was initially concerned.
My main (and basically ONLY use) of the iPP is for art. "Real" Photoshop is a huge get for me (and other artists) once Adobe releases it next year. The extra RAM on the 1tb model may come in handy...BUT, up until the keynote last week, Adobe had been previewing Photoshop for the iPad on the second gen iPP, which only has 4g of RAM, and it looks like it's running nice and smooth with no hickups (on a slower processor, even!)
My wife was at an Adobe conference today, and they again showed off Photoshop on the 2nd gen iPP, and she said it looked awesome.
The file they previewed at the keynote last week was stupid big (I believe she said it was over 9 feet tall, 150+ layers). Those file sizes (and amount of layers!) are for a select few artists. Most of my files range from 5 layers to 20 and I work with much smaller sized art (comics and prints)
Part of me wants to take my new iPP to Apple tomorrow and trade it for the 1tb version, but I'm fairly certain that for most artists, the 4g of RAM on the "lower tier" models will suit them (and me) just fine.