It's an interesting question.
I think when I was younger, and there were less decks/oracle sets available to me...I was much more inclined to sit down and study a brand new system.
but I also really like those oracles where I can make up my own systems, or style of reading with them.
Illustrations do often play a part in whether something will stick with me or not.
Then again, long ago I remember learning the complicated I Ching (with stalks and coins) and nowadays, I just don't want to bother with that whole process! (give me the Tao Oracle...much easier!)
I am much more for instant gratification, or a system that I can just jump right in!
Of course, I also used to make up my own systems for fortune telling when I was a kid...using chips of wood,(that were once roof tiles for a dollhouse) and drawing designs on them.
I guess I still do that!
Recently, someone gave me a beautiful metal charm oracle with a cloth, and a system only slightly less complicated than the instructions for my Sony video camera. (it's called IVATOS...and it's wonderful!)
And for some reason...the whole thing just clicked instantly for me...and I stopped looking at the book after the first reading.
So...it depends on how well the system is designed...how well the cards are designed, and whether the whole thing makes me excited to read with!