Depends on what your needs and preferences are, but for me, definitely AP Business

Better discounts, and imo, a better payment system - you can pay as you go instead of having eBay bill you at the end of the month, which I personally prefer because I found it irritating to sometimes have my eBay labels taken out of my available funds, and other times from my funding source if I had no available funds.

Only downside is that AP Business won't let you add more than 10 items to one order if you link to eBay - I sell a lot of smaller items and that was a pain, so I just use it unlinked. Many of my buyers are repeats and I  just save their addresses there and do a quick check when they order that it hasn't changed.