I have 30 bitcoins. Should I sell or keep?
It depends... If you really need this money, you should sell, at least one part. But if you don't, I would keep for the years to come because bitcoin's got a huge potential when it will massively be adopted.
Of course, it's a bet to do. If you are able to take such a bet, you should take it. We can't think instead of you. It's you the one who has to do this kind of choice.
Keep it securely in a paper wallet. You can invest 3BTC on cloudmining or in the just-dice bankroll if you want to gamble/invest some of it.
Bank accounts serve a high interest in countries with high inflation but high inflation means the purchasing power of your holdings go down.