Recently flew long haul business on both BA and IB from JFK-LHR return and JFK-MAD return.

Check-In
I think the setup for BA Premium at JFK is one of the nicest in NA. IB use this as well. I had not started a journey at MAD before so perhaps I missed the premium area, but there was nothing noteworthy.

Security
Again JFK has a nice dedicated security area which is used for both BA and IB flights. At LHR I continue to find nothing really premium about the security arrangement for premium passengers. MAD was quite efficient and the operation for transfer passengers was also much better than the nonexistent premium lanes (at least that I noticed) at LHR.

Lounge
BA has done a nice job with its lounges for so long, they are always a pleasure. As has been noted the Elmis Spa is really a gimmick compared to the old MB setup. At JFK I really liked the different "waters" they had on offer including the cucumber, this makes for an excellent mixer. Quite noteworthy was how impressive I found preflight dining at Galleries. Yes, it was very busy, but the quality of the food and selection was really very nice. I say this having been at the LH FCL at JFK a few weeks prior, I thought the BA food was much better (perhaps a wake-up call for LH at JFK). The IB lounges at MAD were also a very pleasant surprise, nice job IB. If I were only accessing a business class lounge and was not on a sleeper service I would actually prefer the IB lounge to Galleries, really nice selection of quality food and beverage including Movenpick Ice Cream. The showers in MAD were also quite nice.

Boarding
BA continues to be weak compared to other carriers I have experienced. I don't understand quite why. I was quite surprised at what a nice setup IB had at MAD. Very distinct separate lanes and separate boarding doors were used, really makes a nice start to the flight.

Cabin
I think that both BA and IB have very, very good business class seats and a nice cabin with little touches like flowers in the bathroom, though IB had nicer amenities and kit by far. What is tough for me to say because I really enjoy BA is that I feel that depending on what seat you can get its almost as if there is CW - and CW +. If one can get a window seat you practically have a mini suite, its really a pleasure. My flights were full and I ended up with aisle seating which I hadn't experienced before. To be succinct, it really was not pleasant, I felt quite exposed and really didn't like not being able to see "out." Personally, I really didn't enjoy the experience at all and didn't get much rest. On IB the privacy is not nearly that which can be had with the BA window seats but the cabin has a pleasant open feel and even in an aisle I was not bothered. I also felt that personally the IB flat seat was somewhat more comfortable/cushy than CW. Entertainment on IB is weak and needs improvement, the BA system also needs improvement compared to a carrier like CX but is much nicer than IB.

Service
Really very pleasant on both airlines, which makes flights feel special. As far as food and beverage it was my perception that IB leads BA in all areas from menu to food quality on board. I will also give the presentation to IB as well, really nice use of actual "linens."

In closing, apologies at not being more detail oriented, I didn't have the time for that. I have always really enjoyed flying with BA, I think they are a leader but I was also quite pleased with what I experienced on IB. I think they make a very nice alternative (or partner, depending on how you look at it)!