If you already have a topbonus account, especially one where the miles will not expire as long as you have gold status, use this (but watch out for expiration... AB miles expire after three years if you don't retain gold or above status, much like LH does it).
You'll earn 25,000 redeemable miles with AB for this flight, as well as 25,000 status miles. Halfway to Platinum, or retaining Gold another year... (assuming FRA-HKG-BNE at 20k miles return, plus 25% COS bonus)
Airberlin has "
Schnäppchenticket" sales from time to time - bargain redemptions which provide exceptional value - provided you can use them.
Other than that, I always recommend BA for orphan oneworld flights, simply because BA has the most liberal expiration policies (three years, but any account activity will keep the balance alive for another three years) combined with cheap intra-European redemptions. You will get quite a few RFS redemptions between LON and HAM out of the 25k avios this flight will net you. Additionally, this one flight will net you 560 tier points - almost Silver status (600 is the threshold).
You will have a markedly better airport experience on BA flights from/through LHR as oneworld status holder, doesn't matter if AB or BA. Fast Track security (Emerald only), priority check-in and lounge access make connections in LHR bearable. BA's lounges are a notch above LHs SEN lounges overall.
Also, with oneworld status (again, doesn't matter which airline) you will be able to preselect seats on BA flights free of charge.
If you plan on flying AB: AB extends free XL seat selection (extra legroom seats) only to its own Gold/Platinum members, not to members of other oneworld FFPs holding Sapphire or Emerald status. Major letdown, and they lost out on much of my business because of this.