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.
You must be a day trader then. I am never a fan of day-trading anyway but I understand how the necessity to have a bigger screen comes into play. Being a long term and some terms a short/midterm trader, making use of mobile applications really brings a lot of ease into my trading life.
For the way things have been over time, when it comes to logging into exchanges every time, or having to make use of different exchanges for security reasons, it can make it tiring, but all the same, the fact that I can use applications like tab trader makes things swift.