When all is said and done, I think 27 752s will get the winglets, and it's safe to assume all will be in the International configuration. I believe CO would have all of the 757s scheduled into the North Atlantic market before we ever see a 737 going across the pond.

Don't forget, a 737, like the 757, would require significant modification (not talking about ETOPS, most CO 737NGs came with this certification out-of-the-box) for transatlantic operations, most importantly a reconfiguring of seats to include a BusinessFirst section. The issue with this is that a critical mass of overall seats would be lost to sustain what definitely would be a very high breakeven load factor, leaving such a small margin for profitability that a 757 on the route would make much more sense as it affords more "breathing room". Additionally, the 737 would offer minimal space for cargo, which is often the difference between profit and loss on long haul segments.

In my mind, the only sustainable way to operate a 737 transatlantic is in a single-class arrangement, either a premium offering like PrivatAir or an all-coach product. The aircraft simply does not have enough floor space to make two traditional long-haul classes work economically. The aircraft was not designed with transatlantic performance in mind, and operations on the extreme high end of the 737's envelope would not come without penalty. These flights fall far better into the realm of the 757, and this is why we see the aircraft perform so well for CO in this market.