A few things to point out, and keep in mind.
Vs. threads are very much against the forum rules. Having said that, I believe that while the question is in the form of a Vs. thread, that is not really what the OP is trying to ask.
Right now the RAN and ADF are definitely doing one thing, which is expanding the amphibious capability (to a degree, depending on if/what/how the LCH's get replaced). The second thing which might be happening is the replacement and/or expansion of the RAN submarine capability.
In order to understand what is happening with these capabilities for the ADF, one has to keep in mind that they both provide a number of different potential capability options to the ADF and thus AustGov.
For submarines they are both offensive and defensive naval assets, able to require a disproportionate force response to properly counter. They are also very valuable ISR assets. Potentially hostile countries need to consider if the RAN might send a sub against a naval task force, to mine a harbour, lauch a barrage of land-attack missiles (if the future RAN sub has such a capabilty) or land/retrieve SASR or similar assets.
The types of capabilities the Canberra-class LHD's provide is completely different, yet also of great value to the RAN, ADF, and Australia as a whole. First, the provide an enhanced capability for Australia to lift, transport, and land a large number of troops across a great distance and support them. Given relatively recent ADF history, specifically the deployment to East Timor, Australia realized that it did not have the capability to deploy troops overseas to the degree that it wanted/felt it should have. Especially for deployments within the region. A pair of LPA's were acquired and refitted, but more was still wanted/needed. There is also a whole range of HADR and command/control-type capabilities that ships like LHD's can provide. Given what Australia has been involved with over the last 15 years, and is likely to continue being involved with, expanding and improving on those capabilities is also going on.
The RAN is naturally tasked with different mission sets, which require different types of kit to complete. If the RAN were to only acquire a single type of ship (even a multi-role one) then a significant portion of mission sets would not be able to be completed successfully. For a land-based comparison, it would be sort of like arguing that an Army should only ever purchase tanks, and no other type of combat vehicle. The reality is that without also having supporting APC's, IFV's, SPG's, etc. an all-tank combat vehicle force is only effective against other tanks, or when able to engage hostile vehicles with personnel mounted/on-board.