When you do Law with English Law you will be taught both the Scots and English law, you will do the standard prescribed Scots law course with additional English elements taught in separate lectures alongside. I believe there is no say in this matter, when you pick this particular course you are given all the available English Law elements by default. Where this might differ is in your 3rd and 4th year when you take on honours courses although certain areas of law are the same as each other.
I would say the workload is manageable, your second year is when your English courses tend to kick in and this almost doubles your workload. The English courses give you less credits but you're expected to do the same amount of work as you would do for your standard full credit course.
The university has a lot of law based extra curricular activities, I was part of the Aberdeen Law Project which aims to close the gap in the access to justice within the city and wider area, it's a great thing to be involved in and can give you hands on experience in dealing with clients and even going to court in select circumstances. We also have your more typical focused areas such as the commercial law society and so on. We also have a mooting society, so much trials as you stated, I have not taken part in this before but I can see how this would be a useful thing to do especially as a module you will do in a class in your later years will require you advocate for a client. That being said the mooting society seem to constantly lose to RGU, the universities rivals, so read in to that as much as you like.