"As long as we are better."
Translation:Sofern wir besser sind.
This link gives an explanation of how subordinating conjunctions like these affect German sentence structure http://german.about.com/library/weekly/aa010910b.htm
I would call them semi-synonyms. In several sentences that use "As long as..." one could use either sofern or solange. However, 'sofern' carries more the 'provided, inasmuch, etc.', while 'solange' would be 'while, whilst'.
Ex: If it is raining and we choose to stay put during the downpour, I'd choose, "Solange es regnet, bleiben wir hier".
In the Duo example we're discussing, either word works for the translation. However, if we got really picky with semantics, "sofern" might have more the feeling of "since (inasmuch as) we're better..." while "solange" might be more "as long as (time, duration) we are better". A lot of times, what will help in the choice is the remainder of the sentence.
"Solang" can be also used for time but then a "wie" is often added. But I'm right now not sure if the "wie" is needed. e.g. "Solang wir gewinnen, bleiben wir die Besten" is "Provided that we win, we'll remain the best". Now let's add the "wie", "Solange wie wir gewinnen, ...". This translates to something like "For the amount of time we win, we'll remain the best". You can use "Sofern" only like "provided" and not like "Solange wie".