Not sure if I’m too late, literally just registered to answer this question.
As you are close by and at the current price get the Merlin. Sora derailleurs and a better chain ring, I’m not sure how much difference the carbon fork will really make. I also think it’s a better frame you’re more likely to upgrade later on.
I’ve just ordered a Malt 2 Mountain bike from them. Apart from the great price I’d also complement their service, excellent so far and if you look around the web you’ll see the same from many people. Not sure how good it will be in decathlon. I get the feeling if you were out of warranty with Merlin they’d help you to sort it at a fair price, with decathlon, if you’re out of warranty you’re out of luck.