If I remember correctly, the SMA3800v is one of their older models (heavy LF transformer) so the efficiency will be lower than the transformerless Fronius. I'll also hazard a guess than the SMA will only take 2 strings whereas the Fronius will only take one. Which is best will depend on the specifics of the installation (shading etc.). If your array is shade free, go for the Fronius - newer, more efficient technology (and a darned good inverter to boot)
The max DC input rating of the inverter does not figure in DNO or FIT requirements. At 3.9kWp, your system falls under the 4kW limit for maximum FIT and with the inverter limited to 16A output, your DNO will be happy.