Keep both.
Decide to use the Sony.
Tell signif other you got Koby for her/him, as a Christmas/Birthday present (whichever's sooner)as you thought of her/him as soon as you saw it, and are so excited you can't possibly wait 'till then to give it to her/him.
He/she will be entranced at your thoughtfulness (big brownie points), will cover you with kisses and gratitude (BIGGer brownie points, and a Good Thing), will instantly feel obliged to get
you even more impressive Birthday/Christmas present (HUGE brownie points, and A Very Good Thing). So, emotional profit and well-being all round.
Then, later,
if he/she goes off it, can't get on with it, etc.. sell Kobo, buy her/him bunch of flowers with what you will say is the meagre return (reasonable brownie point/s but still worth having)..........................
With the surplus you will make, put it with the price you will make if you sell the 350, and get a 650 !
Or, the environmentally friendly solution, and a lot of fun for both,
followed by lots of significant othering................