I'm not going to go into detail - Techly's review does a good job and matches my experience.
Firstly - Techly's review mentions that Marley Spoon has limited delivery hours. I picked the 6-9am delivery slot, because I wanted to make sure I was home and the box didn't end up sitting on the street for hours - but it ended up being delivered at 2:35am and sitting on the street (in the rain!) for hours. MS have updated their website to advise that the delivery window is now 2-8am, and I've asked them to call me whatever time of night they leave the box - we'll see how that goes tomorrow.
Secondly: Techly's review describes MS's recipe cards as "easily the best to follow". I haven't found that to be the case.
Partly this is because HelloFresh has a really nice iOS app with very clear step-by-step instructions (including timers for any of the steps that need to be timed), while Marley Spoon only has the paper instructions, and they are often a jumble. They have to fit everything into 6 steps, so one single step might have several unrelated actions in it - "Bring some water to the boil. Meanwhile, fry the fennel". They overlook kitchen implements (the recipe I cooked last night needs a stick mixer, but this isn't mentioned in the list of implements) and ingredients (a stir fry came with oyster sauce and five spice powder, but the recipe didn't mention using them - I had to make my own guess about when to add them).
Still, on the whole, I think I enjoyed Marley Spoon a little more. The recipes are a little bit more of a challenge, which I like (HelloFresh is probably better for beginners) and more varied. HelloFresh is still good though, and I'm keeping my subscription active (but on pause) for now - I plan to swap between the two every now and then.
Both services allow you to opt-out of individual weeks, so I'm signed up to both at the moment but can choose which I get each week. Marley Spoon offers your choice of 3 (or more, if you pay for more) recipes out of their selection of 7 each week; while HelloFresh gives you 3 fixed choices.
BTW, if you want to try out HelloFresh, use the referral code BW2MLZ - you get $35 off your first box, and I get $20 off my next box. Marley Spoon don't seem to offer any kind of referral program.