Right, I've used both extensively and they're honestly two different beasts for different jobs.
The BMV-712 is your traditional all-rounder. Solid shunt-based monitoring, gives you proper coulomb counting, and the display is genuinely useful if you're running around the property checking status. The relay function is handy for load disconnection at low SOC. I've had mine for four years without drama. Main drawback: it's older tech, takes up panel space, and you're tethered to that physical readout.
The SmartShunt is the modern option if you're building around a GX device or VRM portal. Zero physical display (app-based only), wireless connectivity, integrates seamlessly with Cerbo GX or Multiplus. I switched my second system to one last year specifically because I wanted remote monitoring via VRM. The accuracy is identical to the BMV, arguably better integration with other Victron gear.
The real question: are you Victron-centric already? If you're running a Multiplus or Cerbo, SmartShunt makes sense — you get system-wide visibility through one interface. If you want standalone simplicity and don't mind (or prefer) a physical display, BMV-712 is bulletproof and well-established.
Cost difference is negligible. SmartShunt edges it for new builds; BMV for retrofits where you want display redundancy.
What's your wider system looking like? That'll probably determine which suits you better. Worth mentioning too — some folk run both (shunt-only monitoring plus the main meter), though that's probably overthinking it for most setups.