My key advice is re-implement rather than upgrade. Then you can take full advantage of the new frameworks particularly for COA dimensions and WMS. The work, pain and cost to convert legacy date is just not worth it and is better invested in the future than the past.
Ax2012 R3 will be around for at least another 4 years and as most of the functionality is common then I would expect ongoing updates to Ax 2012.
There are advantages to a cloud deployment and disadvantages.
You have to consider how much the new technology stack can benefit you e.g CRM features.. How much will a web ui multi platform system benefit your particular industry?y e.g. how many user are mobile, or office based?.
You may already have sunk costs in licenses and hardware that you cannot easily write off.
You will save BREP, electricity etc. immediately on the cloud but you will also have an immediate, and on going subscription cost to pay - and its named user not concurrent with a minimum of 20 enterprise users so even with a discount against your existing licenses this will be an immediate cost to bear. But no more AOS to buy, no need to buy a store server every time you open a new retail outlet, low cost team users.
Do you use Office 365 ? If so then a seamless experience with DMO365 will be attractive.
How much data needs to go to the web? How often do you update data? what volume of transactions'. Do you have heavy month end printing loads e.g. utility company.
You may also have additional costs to use the cloud - internet bandwidth, azure express route to reduce latency, additional environments for test, storage space for back ups etc.
Another factor is how much customisation and interfacing. Ax 2012 R3 considerably changed the database table structures and introduced the record id. In DMO365 you don't have direct access to the Production server so code moves have to be managed by coordination with Microsoft, and that will not always be at your convenience. If you have diverse LOBs, and multi tax regimes, payrolls then you may need to regularly update code.
At Ax 2012 licenses were changed to named user with roles tied to security, and are subject to audit and you will need time to get your brain around how that differs from Ax 2009.
don't forget to also look at compatibility with e.g.SQL versions, WIndows versions etc. If on premise and re-implementing then also look at beefing up your server, and upgrading SQL database.
Don't forget to allow time for training and testing there are many additional features introduced at Ax 2012/DMO3654, e.g WMS, Call Centre, Process Manufacture......
Microsoft clearly has a strategy to move all customers to the cloud so expect them to encourage that, but they can still guide you through the licensing etc. Its already starting to sound complex, and it is, and you should discuss in depth with a reputable partner.
There are new security challenges with a web ui - so the Ax7/DMO365 Ax on premise story changed last year from azure technology stack to - some more limited offline synchronization for some features only - there at hints that a new announcement will be made around June about the option to have a private cloud with option whether or not to sync data.
