Hi I'm a bit late to this discussion, my 2 cents is that inverter capasitors are the limiting factor for longevity lasting for about 15years the current inverter is 5years already ⅓ of its life about. If budget is no option then 10k inverter would make most sense as it refreshes the life of the inverter provided you get the remaining roi back for the current one. When the current one fails of you have 2 then parallel leaves you with one you again need to pair out of roi sync. Parallel makes sense if you just need extra capacity with minimal outlay just add the inverter, connect to current paralleled batteries add one breaker in the AC box & may be a termination block to parallel the AC grid inputs outputs of the two inverters no more spd or seporate circuits etc, setup master slave, simple. It does, as mentioned, by others, give you extra mppt's. It will drive your batteries harder so you would need to check that batteries are comfortable with the extra max draw.