There might be more benefits if you work for Royal Caribbean because then you are actually employed by the cruise line. Harding Brothers are a concession so you are not employed by cruise line so lose certain benefits like being able to get discounts for friends and family after a certain amount of months worked. Sometimes concessions have to pay a certain percentage of salary towards living costs onboard like food too.
Another consideration is salary, I know guest services on RCCL get around $1620 a month and also get their flight ticket home paid once they complete their contract of 5-6 months. There are definitely chances for progression at RCCL once you have completed one contract and they have restructured guest services so there are more achievable steps to progress to. The training is a bit longer on RCCL but all takes place onboard - you can expect most of your first 2 weeks to be training, it''s a lot to take in but they take it seriously. Depending on the cruise line you get put on with Harding Brothers you will still have to do training and length could be longer or shorter than RCCL's 2 weeks. Both positions mean you share a cabin so no difference there.
Lastly, I know that guest services work around 8-9 hours a day, usually if you start early you get time off in day then finish around 8pm or if you start later you get time off in the evening and can work until midnight or 1am. Shops do slightly more hours I think and usually finish after midnight a lot of nights. One benefit of shops though is that the shops can't open in port so you will get almost all the port days off.
Anyway, hope that helps. It's a tough choice and going to Sydney sounds cool so good luck with whatever you pick!