You can buy Jonin Minato for a very reasonable price of around 5-7k coupons from this week's Sakura event, and I strongly suggest that you do. You'll be able to get Shisui for around 12-16k in the future, but I can't guarantee from which event exactly as they tend to change quite frequently.

When it comes to choosing which one is better, it's mostly up to your taste, unless you go for both. They're both great standalone characters that work with multiple mains and different team variations