ESS/MSS Activate SAPUI5 or WDA on HR Renewal feature 1

Hi Experts,

I wonder if you can help. We have upgraded to EHP 6 and we are on release 604 SP82. We have SAP renewal 1 and SAP Netweaver EHP3 for 7. See the attachment for exact details on our system status.

My Question are:

1. Should we switch on services for SAPUI5 or Webdynpro ABAP? We are already using webdynpro ABAP for ESS/MSS. I mean SAPUI5 is better on the look and feel but I am not clear on any technical requirements that have to be considered before heading the UI5 route.
2. If we switch SAPUI5 can we go back to WDA at a later stage if we see that we have a lot of issues during testing? ie can we reverse SAPUI5 to WDA
3. If we decide to go down the WDA route, what functions in the portal do we need? I know which ones to activate in the backend but not sure on portal side.

Your input will be greatly appreciated indeed.

Regards,

Andy