Desktop of course. Is it even feasible to trade on a mobile device? The screen is too small there's no way you can read info (orderbook, charts, volume, etc) properly. Even with a 14-inch laptop I often wish my laptop had a bigger screen.
The platform doesn't matter much to safety I think. Just make sure you set up your independent 2FA properly and you should be safe.
Thanks for the answer but I think it s feasible to trade since there are many trading apps ?
Yes, there are many trading apps with the most common one being tab trader. Personally, I only do few trades on my PC making use of Coinigy with my bittrex API on it to probably do some extra analysis if I want to get into a particular market which I need more conviction about.
Moreover, mobile applications like the one I have mentioned have been my best friend since I can easily get alert to stop loss without being on my system every time, I can make any single trade as well as analysis easily on it, and the fun part, it is mobile (on-the-go).