I would go with the 900 but he 680 is adequate...
I have been running the 680 for several years and more than 600 hours and it is adequate for a 390 but if I had it to do over again I would go with the 900 for the extra reserve and depth of charge.
RV7 powered by a lycoming thunderbolt IO-390
turning a whirlwind HRT prop
flying with more hours flying than building... 2,260 on the hobbs!