Voltorb flip. I like games that make you think a little, so it was easy for me to spend hours playing Voltorb flip. I never got hooked on the slots in the same way. It was fun to figure out the strategy to easily win with Voltorb flip by myself, but of course there's also the luck factor, which makes the game not too easy. Plus, it was easy to get way more coins from Voltorb flip than I ever got from the slots. With slots, I always ended up losing all my coins. xD
The slots are definitely fun and nostalgic, and the option to buy coins was a cool addition. However, I don't tend to like things that are based in luck, and Voltorb Flip actually gets you to focus on it and think about your strategy. It is much more engaging and fun than the slots, and it is much easier and faster to get coins. I've gotten to Level 7, where the payout becomes insane. I have never really played anything like Minesweeper before, but Voltorb Flip is still fun. Of course, this is just me.
The slots were awesome (and the roulette was even cooler, but we tend to forget Ruby/Saph don't we?). I liked that you could buy coins, because I don't think anyone actually took the time to save up coins through winning the slots.
That said, as far as the actual game goes... voltorb flip is better. I got really good at it and ended up getting my friends to trade me some of their pokemon in return for winning them a Dratini or something else.