I have 250 ETH, Hold or SELL?
250 ETH are a good portion. 
When ETH comes to PoS you can easily make shitloads of money for staking your ETH. Minimum amount is 32 ETH, you have well above. Congrats, you'll be very rich.
I don`t know I hold or SELL.
I would hold, you can make PoS if it's ready.