This is going to be difficult...
Can I assume that you've found these laptops on pcbox.com (a Spanish site)? And can I further assume that you're either Spanish yourself or speak Spanish?
If yes to the above, then I have the following comments:
The Lenovo Legion Y520 is a very decent and affordable gaming rig. By all accounts you'll get good specs and decent build-quality. However, most users have reported absolutely horrible battery life (30 to 60 minutes of actual gaming).
Lenovo are fairly 'late to the game' in the gaming business, and this may explain why their models are often less expensive than more established gaming brands, such as MSI and ASUS ROG. The Legion models (Y520 and Y720) also suffer from poor heat dissipation design.
In contrast, the ASUS ROG line is a well-established and generally well-respected brand offering the full range of high-end specifications, including 1080 GPUs and IPS panels - you can't get these features in any Legion model.
But take a few minutes and read this notebookcheck review (in Spanish) from March 2017:
https/www.notebookcheck.org/Breve-analisis-del-portatil-Asus-ROG-Strix-GL553VD-7700HQ-FHD-GTX-1050.207244.0.html
NOTE: The model tested features an i7-7700HQ CPU and an IPS panel. The reviewers generally like the ASUS but they also make the same observation as you do about the 'added' 100€ (compared with similar MSIs and Dells).
In your case, the added 140€ might be justified by the 4-cell 48Whr battery which is likely to offer significantly better battery life than the 3-cell 45Whr unit in the Y520. Other than that, I agree that the laptops are basically identical.
If you'd like an opinion as to which one to get, I'd say go for the more established ASUS machine. While I don't know exactly how much battery life you'll get from the ASUS, it's bound to be more than from the Legion.
Happy gaming,
GreyCatz.