The actual achieved rate of climb depends upon many factors and when flying with passengers one of the most important ones is passenger comfort often referred to as deck angle.
If you climb a Seneca at 1500 ft/min then the pilot will have no forward view and no view of other possible conflict with traffic from the 10 -2 o'clock positions. On VFR departures I always prefer to see where I am going.
The aft facing passengers will be hanging in their straps.
1500 ft/min is an uncomfortable rate of climb/pressure change for passengers and particularly for the kids who don't know hoe to counter this, anybody with a cold will really suffer.
In the 340 you can keep the rate of change in pressure very low and you can also move around see to the comfort of the passengers and if they feel in flight discomfort move them up front so on a long trip time soon flies by.
The turbo charging system on the the 340 is much superior to that on the Seneca, get the oil warm before take off and you can set full MP on take-off
as they are not prone to overboost.