Although 3-phase inverters are more expensive than single-phase inverters I believe their benefits outweigh the extra cost (expect to pay about $300-500 extra for a 3 phase 5kW inverter compared to a 5kW single phase inverter).
A single inverter across three phases results in more stable operation, with less voltage and frequency swings and less tripping off of the inverter. If the inverter trips you lose all your solar generation until the inverter is manually or automatically reset.
In other words your 3 phase inverter is much less likely than a single phase inverter to trip off on 'over voltage' or 'out of frequency' faults, which is becoming more of an issue as more solar is connected to the grid.
So if you have a choice consider paying extra for a 3 phase inverter.