In the longer term, establishing your career and beginning to get experience will be the most important factor.
That said, my own recommendation would be to go for a level 4 apprenticeship if possible. This provides a bit more
progression academically when looking at the Qualification Framework (QCF).
You already have a level 3 qualification (BTEC) so the only value you'll get from doing a level 3 apprenticeship is from the work experience part. Undertaking a level 3 apprenticeship may be useful if you'd like to move across into something new
like marketing or HR, and is not the end of the world if that's what you choose to do.
Do you have any ideas of what subject or qualification you'd study as part of the level 3 or level 4 apprenticeship? Your
future career ambitions are something to consider, and will certainly influence what would be the 'better' option.
Hope that helps.
P.S: I did a level 4 finance/accounting apprenticeship after completing my A-levels.