I am wrapping up the Finish Kit and would like to know which kit to
tackle next. It appears that either could be accomplished at this point
but curious which would be more efficient.
It can go either way. I think that the avionics package has the greatest chance of getting revised / upgraded over the intervening time so that might best be purchased / go in last. The engine has less of a chance of seeing revisions / upgrades, but sitting there without being able to run doesn't do it any favors. However, I believe that the ROTAX engines are test-run and pickled by the factory so short-term storage / inactivity shouldn't be an issue. Both the engine and avionics packages are relatively easy (and fun!) to install.
Engine next then Avionics. Some of the engine install requires the avionics package so depending on your work pace you will have to leave some engine installation incomplete. Avionics are constantly changing and being upgraded. For a rough guide on when new things come out they are usually announced at Sun & Fun in April and OSH in late July .
Electronics technology changes monthly. My last RV's engine was new in 1961. New slicks, Sliver Hawk FI and new B&C starter and alternator made it nearly new.
When I was in a similar situation I was waited for the Skyview conversion from the D-180. It took longer than I hoped but the techs wanted to get it right before release. It was pretty close to just right. Only a few changes after release, but it was worth it to me.
I moved forward with the engine and did avionics last. No issues with this order.