I was at the University of Pennsylvania, and my home university is Edinburgh. I study Computer Science & AI.
At Penn, I was basically on holiday. I didn't purposely take easy courses, I just massively overestimated the difficulty of courses. It seemed like a lot of my classmates (even those majoring in CS) had a poor background in their subject, and most courses were setup to cater for this. So I found most of the courses easy, even boring. I spent a lot of time trying to find good courses, I sat through many.
There are more assignments during the term, but they are mostly 'busy work', so assignments just for the sake of assignments. For example , a programming assignment on something not really relevant to the course. Almost every course will have a group work component. It can sometimes be hard depending on your group, and you need to prepare presentations of work (even for CS courses). There are midterms, but the homework counts more, and the 'exams' are open book and mostly not too serious. So a lot of the focus is on homework and group work, which doesn't always require you to know the material too deeply. You also get *a lot* of help (office hours, TAs, lots of Q/A forums).
After returning to Edinburgh, I found the semester far more stressful. While you may only get 1 or 2 courseworks per course here, they are really hard and require you to learn and do a lot of stuff by yourself, with minimal help. The exams here require you to know everything in a course, you can't get past it with good group members in a group project. Because the education is focussed here, they know what students know in most classes and go through way more advanced material.
Overall I learnt tons more at Edinburgh. My time at Penn really surprised me. Though it's probably harder to get a good undergraduate mark in the US if you don't 'know the system'. For example, I didn't realise 'extra credit' actually counted, so if you want to go an A you need to do all of these weird extra credit things. Grading is also curved, so you have to do the homeworks absolutely perfectly. If you lose 5 marks for not naming your files correctly, it can mean no A. This also means having your group for groupwork prepared before the assignment - if you don't know good people in the class, things can be bad. Also, if a professor doesn't like you, he can mark you down. I got marked down to a C for a course because a professor had a *very* big argument with my team member on a course, so we both got marked down heavily. It was absurd. The professor refused to provide our mark sheet for the final assignment, it was blatantly obvious that he marked us down simply to get back at us!
The biggest strength of the US system is the ability to try out lots of different subjects. But you will be at best a jack of trades. If you want to go deep in a subject, the UK is great. The standardisation, anonymity, and external examination in the UK is also something that shouldn't be taken for granted.