Best is just determined by the amount of $$ you have and reachable goals. Both engines work fine. The 7M will cost $$ to address "flaws," and the 2J will cost $$ because of swap costs/work.
If the 7M is still running, I'd keep it while I collect 2J swap parts over time. When the time comes, just pull 7M & swap 2J in, with hopefully little down time. Last thing you want is for either swap to run into problems, leaving your MKIII on jackstands for months, or even years.