As has been mentioned above, I did my ATPL theory with CAPT a few years back, and I heartily recommend them. I have no direct experience of Bristol (other than using their question bank), but from chatting to other candidates before and after exams, it seems that they tend more towards the 'memorise the answer to these questions' approach, rather than getting the student to actually understand the material. Of course, if you're a helicopter pilot studying a bunch of fixed-wing specific stuff, that doesn't help either.
There are more differences between the interim and helicopter-specific courses than you might think, although there are obviously a lot of similarities. In addition to the obvious PoF, the M&B is done on a helicopter rather than a 737 (although the principles are basically the same), and the Nav papers don't include stuff about flying over the Atlantic, for example. There is a lot of material to get through, so making sure that as much of it is as interesting to you as possible will make the whole experience a whole lot less painful.
In terms of personal attention and getting on courses, etc. you're probably better off picking the smaller school, as there's less of a factory atmosphere (I guess). All I can say here is that I had no trouble getting the dates I wanted, and I don't believe this is a problem with either school.
You're paying for more than just the notes; you're paying for a structured course, with progress tests etc., as well as personal help should you need it. You're also paying for several brush-up courses to help prepare you for the exams. And, given the cost of most rotary training, a few thousand pounds isn't really that much. Certainly, I've found the CAPT notes useful well beyond the course, and I wasn't interested in selling them on when I was done (they had some useful stuff for when I started my IR, for example); this is an indication of the quality, in my opinion.
The route I would suggest is to sign up for a course with CAPT, then just before you write exams, get a question bank subscription from Bristol (this used to cost £50 for 3 months access). CAPT will support you going as fast as you like, and if you have the time and determination, you could probably blaze through quite quickly.
If you have any specific questions you'd like to ask about how I found stuff, feel free to msg me.