I feel like Bulgaria has more potential to develop faster cause

( I know its not only about GDP per capita but I am giving you those numbers to give you a perspective)

Romania has 11534 GDP per capita

Romania's neighbors;

Hungary - 16530 ROMANIA - 11534 Bulgaria - 8651 Serbia - 6880 Ukraine - 3110 Moldova - 2684

Bulgaria's neighbors;

Greece - 23558 Turkey - 15026 Romania - 11534 BULGARIA - 8651 Serbia - 6880 N. Macedonia - 5394

So Bulgaria is closer to more developes countries meanwhile Romania's neighbors are usually way less developed than Romania.

Also Bulgaria has less population so their gdp per capita can increase easily.

As I said its not about GDP per capita at all but it also gives you a perspective.