BTEC is more restrictive on the courses you can do at University. The work is completely different as is what determines your overall grade.
I've always had the thought that BTEC is generally harder to get the highest grade (D*D*(D*)) as you need to get a D in almost every piece of work. This is because your grade for a unit is the lowest grade you achieved out of all pieces of work in that unit. You could get all distinctions and get a single pass in each unit. This would give you a pass on each unit and a low overall grade eventhough you got so many distinctions. Where as with A-Levels you have a couple of exams near the end of the year.
That being said, A-Level work is generally considered more stressful and harder as you will be doing two or three A-Levels at the same time.
Another thing about BTECs is that you constantly have assignments due. Although the work is not considered as difficult as A-Level (despite that being decided by the individuals) you have a many deadlines at the same time and they are very strict. If you submit a piece of work on time you only get ten working days to resubmit it, if you get a resub chance at all.
The most important things to consider in my opinion are as follows:
1) Do you prefer exams or coursework?
-If you prefer exam based courses then A-Level is the best choice
-If you prefer coursework heavy courses then BTEC is for you
2) Where do you want to go?
-Do you want to go to University? If so then look at the course you want to do, at various universities and see what qualifications you need
-Do you want an apprentaship? If so look at the qualifications you need for it. You learn in a more vocational way in a BTEC so you will learn various skills required in the job area the BTEC is in.
3) Do not trust what people say about one being better than the other.
-You WILL hear people say that A-Level is better than BTECs and BTECs are easier and you will be working in a fast food place or in a shop with them. This is not true. Both are pretty much equal, each having their own pros and cons. ALWAYS trust what careers people or people with more knowledge and experience say above your peers but never 100%. Always take it with a grain of salt.
Finally, it is 100% about you. What suits you is what you should do. Do your research before you commit.
I hope I have been of some help although I diverted from your original post. If you have any queerys you can ask me and I will try to help as best I can