I prefer offline wallets. Offline wallets are the best option if you want to hold a large amount of bitcoins. The wallet is safe and you have full responsibility for it . There is no chance of facing a cyber attack and loosing all your assets.  Whereas , there is always a risk of getting hacked if you are using a online wallet. A single hack on the site of online wallet can lead to a huge loss for you. But online wallets can be used anywhere which is a very good feature fo traders so we can keep a less amount of bitcoins in the offline wallet.