24v vs 48v solar inverter
The advantage of 48V over 24V is that only half as much current is required to get the same power. Assuming 95% converter efficiency, for 3kW output at 24V your battery wiring has to handle 132A! At 48V it drops to a more reasonable 66A. This is actually better than you might think because power loss is proportional to current squared, so if you use your existing wiring and connectors the loss in them will be 4 times higher.

A 24V inverter might be a bit cheaper, but you should consider the cost of replacing your wiring and fuses etc. You should also consider that a cheap 24V inverter might not be as reliable as a higher quality 48V unit.