I've had experience of doing both AS (biology,chemistry, geography, phil & ethics). With AS I worked constantly making notes an doing the homework but I fell ill the first couple of months into the academic year which really made me struggle. To cut a long story short I failed all my subjects apart from geography.
In order to make me better I (along with my doctor) decided a change of scenery would be the best option and so I applied to do a btec in medical science (applied science) and have successfully completed year one obtaining D* D*
All these comments about BTECS being 'easy' are insulting to anyone undertaking one. I have never put so much effort and work into something in my life, I have worked far harder this year than I have doing AS, this past semester I have worked solidly all day everyday (5am-1am) to meet very tight deadlines. The problem I find with a levels is that it is too structured, the modules (particularly the biology ones) are rather pointless and will not help you in the future. With a levels you get spoon fed absolutely everything, your teacher holds your hand throughout experiments and most of the time it is all set up for you ready for when you arrive, with btecs (well I have found) it's very much an 'off you go then' mentality and we are expected to do everything ourselves. One of the units in the first semester involved us designing, undertaking and writing up our own experiments without any assistance at all. My experiment involved isolating antibiotic producing colonies from soil samples which I would not have been given the opportunity to do in sixth form because it simply would not not have fitted in with the syllabus.
After a year I can write well structed and factually correct lab reports (the kind you'd need to produce at uni), confidently and accurately undertake complex experiments, use an automatic pipette (many of my friends going on from sixth form to do biology in September have no idea what these are) and use more complex experimental techniques. Would I be able to have done any of this in sixth form? No, and this gives an advantage for when I do go to uni
In a nutshell, btecs can be as hard as you wan them to be. If you don't care then you'll find it easy putting minimal effort in but if you do care, it can be tough, much tougher than the strict syllabuses of a levels that do not allow you to build up an in depth knowledge of areas surrounding your field of interest as your main priorities are memorising the syllabus in order to be somewhat successful in your exams.