Is pre-mixed ethanol-free fuel better than mixing yourself?
Nothing is guaranteed but I think the odds of you messing up the fuel/oil mixture yourself are greater than getting a bad batch from the factory. So I would say yes, the pre-mixed fuel is a safer bet. Also if you buy pre-mixed fuel there is less risk of water/dirt getting into your gas cans.

However I personally think it's not worth it. Buying pre-mixed fuel removes one possible source of problems but it does not guarantee trouble-free use year after year. You will still want to perform basic small-engine maintenance like emptying the fuel tank at the end of the season, checking the air filter periodically, etc.

You could do some back-of-the-envelope math to see how much it would cost you to use the pre-mixed stuff and then compare that to the cost of a new trimmer if this one does bite the dust. At a cost of $20-30/gallon it could really add up if you use the trimmer regularly.