I did extensive research on this as I am in the midst of installing a 200W setup on my AS.
The answer is it somewhat depends as there is a convolution of factors, and as always, it's dependent on variables for your specific setup. Though things seem to favor PARALLEL more often than serial for MPPT controllers (PWM can only do parallel). Which is supported by professions from Lewster to AMSolar whom recommend parallel wiring
One of the common reasons for serial I've read on the boards is wiring gauge. True, higher voltage increases transmission efficiency over smaller gauge wire. But rarely talked about is this:
1) The greater discrepancy between array and battery voltage, the bigger reduction of the conversion efficiency of the controller.
2) MPPT controller efficiency tends to drop way off at lower currents, which is another strong reason to wire in parallel.
Here's my particular controller, but you can see the general efficiency trends:
For 200W, wiring absolutely should be parallel.
For 400W, if on really wants to use the 10gauge pre-wire, I can see potentially a series/parallel wiring of the panels to balance the tradeoffs. Optimally, it should be all parallel with upgraded wiring.