The reason the other university is in top 5 isn't necessarily because it's harder but because they teach better and that is also why it's better for employability also, I think the top 5 also means that people tend to do well at that uni.
Also also, every uni teaches roughly the same thing because there are certain things they have to teach in order for your degree to be valid, therefore, I think once you get far enough into the degree it will be equally hard at both universities but one will provide you with more opportunities than the other. It's normal to feel out of depth but have a bit more confidence in yourself, if you meet their offer in your exams then you are capable of doing well with them. Don't miss out on opportunities due to doubt. So, I say as long as that's the only concern you have about university then go for it.
Good luck 🍀 🍀 🍀 regardless of what you decide.