I mostly use a smaller knife for most things. But I know the appeal that big/bigger knives have. I don't think you can go wrong with either knife. My first BK was a BK-2. I quickly decided it was too much of a hunk of steel for me to want to carry it. Then I went smaller (BK-15, BK-16, BK-14, BK-24). Picked up a BK-7 which I like. It is still a big knife or at least it is to me. I don't need a big chopper since I will use a short machete for that task, so I have stayed with the BK-7. If I have chopping, the BK-7 gets left at home and I take a short Condor machete like the Pack Golok and a smaller lighter fixed blade.
If chopping is a dominant use and you are comfortable using a larger knife for semi-precise cutting, I'd get the BK-9. If not, I'd go with the BK-7 and get the BK-9 later. It doesn't make a lot of difference as long as you are comfortable with using a larger knife. Either of those knives are a good practical choice for a larger fixed blade.