Some of the other toughts are that printed cards are guaranteed to be from the casino while blank cards could have been acquired from the card manufacturer and may not have even been used in the casino. That happened in the past when many of the casinos in Montana stopped using slot cards and someone acquired some or most of the remaining cards from the manufacturer according to rumors.
Plus some collectors collect the different player number prefixes on each issue cards like with the Harrah's Total Rewards cards, Isle of Capri or others.
Some collectors just collect the different fronts and ignore any changes on the back, cards from only certain casinos or geographic areas or whatever suites their fancy.
Personally, I collect just about every different variation I can get... blank and printed, player# prefixes, different magnetic stripe colors and sizes, different stickers or special hole punches for seniors, VIP or others; and many more.
It all depends on the collector and their interests.
If you have any specific questions just let me know. I publish the Slot Card Guide that's a free download from my website at www.slotcardguide.com and I'm the slot card admin for the ChipGuide.