Voltorb Flip all the way. Slots were boring, based more on luck and could leave you with less coins than you had when you started if you weren't skilled enough to play them properly. I would have never made a PC clone of the slots.
I'm not a fan of both, sort of neutral I think about it. Though I think it would've been nice to have both in the game, sort of like Hoenn's game corner. But what would've been nice would have been to be able to buy the coins like in the original.
I prefer the slots over Voltorb Flip. Voltorb Flip is like Minesweeper and Picross had made a baby and produced that. -_- I happen to be terrible at both, so Voltorb Flip isn't fun for me. I'm glad it's free to play, otherwise I would have lost a ton of money.
I don't like the fact that the slots were censored, or that the choice to buy coins was removed. However, Voltorb Flip is fun, and I think they should've included it in adddition to the slots... not just one or the other.
One good thing about Voltorb Flip; it doesn't take long to max out your coins if you're good at it. I maxed out my coin case on my second day playing it.
Honestly, the very notion of Voltorb Flip used to make me see red. Then I warmed up to it...then I got annoyed again when I couldn't figure out how to play it...but now that I have more or less mastered the art of playing it, I love it. I really like the element of strategy and the fact that you can't lose the coins you've won. I do sometimes turn on my older games just to play the slots, though.