Geauxtigers

*****

gkl

1. https://www.oneworld.com/ffp/extra-baggage-allowance
unfortunately this one is not very clear. On the one hand only BA´s handbaggage only fares are excluded, on the other hand international flights with "piece" system allow a second bag, only when the ticketed allowance is already one bag. I could not give a clear answer on this one, from a gut feeling I´d say that OW sapphire receive the same benefits as AB´s own goldies (i.e. an extra bag even in JustFly), but don´t quote me on this. Maybe another FTer has already an experience.
2. There are check-in counters in Terminal C which are always open during the day.
3. no, standby for an earlier flight is not something which AB offers as a free benefit. You need to buy a Flex ticket (which also offers a free checked bag - however it might be a hassle, when you check in the bag for a later flight and then want to swap to the earlier one....). To my knowledge this benefit is also not offered by SAS.
4. yes you can spend time at the lounges (either in the more-or-less equal AF or BA lounge in terminal A), but I would not return earlier from the city just to spend more time than to bridge buffer time until boarding.

SK2751

A few extra points:

SK does not offer earlier bag drop off either. In fact, since this is an outstation, check in tends to open late (I have been burnt waiting to get a BP before in TXL).

Neither AB nor SK offer free stand by. This is in general almost never seen in Europe. Free standby for earlier flights is typical of US airlines (sadly so).

Lounges in TXL are quite bad. I would totally prefer to kill the time in a bar or restaurant in town. TXL-CPH will depart from terminal C (if on AB). There is no "lounge" airside, just a waiting area with free coffee and soda. Not particularly pleasant, but more quiet than the rest of the terminal (it also has wifi). [but, someone correct me here, I think that non-AB elites have no access]. You can choose to use the lounges in the main terminal.

If you fly SK PLUS, you might get access to the Lufthansa lounge (but you need a boarding pass, and the lounge is also land-side, in the main terminal, not in the building that SK usually uses). [at least this was the status when I used it a couple of years ago]

I tend to use TXL quite regularly, and like it despite rather spartan services. I think it is a fairly efficient airport, so there is little need for better services. Just do not plan to spend there a lot of time.

AlienInTheFatherland

If you go for a cheap flight with AB it is unlikely that you will accrue any miles or points. Therfore you might also want to consider easyjet who fly SXF-CPH up to three times a day for as little as € 23 one-way. Of course you would have to pay extra for checked luggage. SXF (Schönefeld airport to the South East of the city, currently using the same runways as the ill-fated new Berlin airport BER) is easily accessible from town on a direct train service (ca. 20 mins).

Concerto

EasyJet does, interestingly, offer standby to earlier flights on the same day, assuming there are earlier flights on that city pair. It's the only airline in Europe that does this to my knowledge. However, there are couple of conditions attached which can be found by digging around in their TCs. I think one of them might be that it has to be the inbound of a round trip ticket, not sure though.

cha211

If you look at Easyjet, then also consider Norwegian. They too fly SXF-CPH and they are quite ok considered that they are a LCC.