I take level 3 btec diploma art and design, amd had the same option as you to do that or a level art. I decided upon this choice for a few reasons:
-i wanted a range of options to try such as photography, model making, painting, fabrics etc which the btec provides by giving different units with different requirements to choose from. For example you can do a graphics unit or a 3d design unit depending upon your interests and skill levels.
-the diploma is equivalent to 2 a levels. In my sixth form we have to do minimum 3 subjects so i do the diploma and then a level English Literature alongside. This has helped me for university since I want to go into theatre design.
-btec is highly looked upon for art schools and creative universities. Sure, I was told by a Cambridge lecturer that I'd never get into Oxbridge (she came to the sixth form to talk to us) but then they don't particularly do creative subjects. However, every university I have visited has said I don't need a foundation year as btec level 3 diploma is enough experience.
-with that, a level is sometimes considered too exam based for creative subjects like art and design. Therefore, unis see that the btec is more physical experience alongside the written and research methods- aka it's more like uni. They like it a lot for that reason.
-Leeds told a classmate of mine that if she had done a level graphics then they would not consider her at all. However, since she did diploma art and design (heavily focussing upon graphic design within that) they have given her an offer.
-within my class we have graphic designers, illustrators, animators, model makers/3d/prop/set designers (me), artists and fashion designers. We are all taking the same units (aside from the afirementioned choice units which you choose which to do) and same course yet we tailor it to our interests. This is different to the rigour of a level art.
-i took gcse art and gcse graphics. Both have helped me for my btec- art with the traditional art knowledge and graphics with the coursework style, but the gcse graphics was way too exam focussed for a creative subject. To the extent that my school now teaches btec level 2 art and design instead.
Hope something in that ramble helps!!! Sorry for writing so so much!