A few weeks ago I tested out LingQ quite excessively (with the result of deleting my LingQ account and subscribing to Readlang instead), so I will add my thoughts here as well.
Reading experience
I’m first and foremost a book reader so I really appreciate that Readlang lets me read texts as they were written and does not chop them into “lessons” as LingQ does. The “Explain” feature is also far superior to LingQ’s dictionaries and I love how an external site like Wiktionary is seemlessly integrated.
Practice
The Readlang practice modes are fun and I quite like that they go beyond just presenting flash cards.
LingQ seems to only save a small part of the context and I believe it isn’t editable, whereas I feel that I have a lot of freedom on Readlang about what to save for my word list, which is great.
EDIT Forgot to mention that I love that the words get ordered by frequency. It’s not perfect but it works well enough, so that I can collect all the most obscure words without feeling guilty 
Library
The LingQ library scared me off the most. At best it is distracting, at worst anxiety-inducing (I really don’t want to get forced to read disturbing newspaper headlines).
The Readlang library is clean and unobtrusive and doesn’t distract me from learning what I actually want to learn.
The Storybot is a really nice addition, which I appreciate a lot for more niche languages.
Known words
I have to admit that I rather like the collection of known words. It seems to trigger some hunter and gatherer instincts in me and felt quite motivating. But this does not outweigh everything I found negative on LingQ, so I’m fine to live without it.
User friendliness
I think Readlang is a lot more user friendly both in terms of support and subscription models.
The “free” plan on LingQ is a joke (I was only able to test it properly because I signed up with an affialiate link, so I got more than just 20 LingQs) whereas Readlang’s free plan is actually usable. This makes me a lot happier to spend my money here.