Britan for a few reasons (listed from least to most important):
1. Germany didn't have great long range heavy bombers. Not on the order of the B-17, B-24, Lancaster, etc. anyway. This would limit their effectiveness on a per sortie basis.
2. Similar to in AH, the British could RTB, re-fuel, re-arm and re-up relatively quickly while the Germans faced a much longer turn-around time. This means the Brits could effectively do more with less.
3. As round 2 continued, the US would have entered the war and come to Britan's aid.