I think it really depends on which you find most interesting, look at each of the topics in the syllabs and see if any 'jump out' and perhaps do a little reading on the net to provide further basis for your decision.
For my HSC, I did WWI -> Germany -> Riefenstahl -> Conflict in Europe; which are all very 'German' based, and thus information overlapped and concepts did become a bit repetitive. Though this was a double edged sword so to speak, cause over the year the same events were referenced, the same people, the same issues, the same results, the same stuff really; so things got really repetitive after a while. However, this can be a blessing, as when it comes down to remembering information and studying for the big exams; trials and the HSC, there is seemingly less to remember.
Germany was a great topic to study, and I loved majority of it; the politics did confuse me a little to be honest, but it was broadly a fun and interesting topic. Though, I have heard great things about Russia as well, so really it's up to your personal interests. There is also a plethora of resources available on both topics, as they are both very popular, so you shouldn't have too much finding assistance if need be.