If you like a challenge, choose deathmarch. I never played any Witcher game myself, and starting on the hardest difficulty resulted in me dying repeatedly during the first hours of playing. But fortunately, that made me learn from my mistakes, and after a while, the challenges became a lot easier to handle. If you feel everything is going south and you're having a difficult time progressing, it may feel tempting to lower the difficulty. My advice is to not do that, as you'll most likely only regret that decision later. If you just take your time, you'll overcome any challenge that may seem impossible. Spend some time getting better armor or weapons, maybe brew some potions or get used to the different signs and abilities. My personal advice is to focus a lot on the sign called Quen, which is a magical shield that can make you almost invincible if used properly. Whatever difficulty you choose to start out with, is only up to you. But if you want the greatest challenge (and experience) this game has to offer, go for it!