Negative/positive ground solar charge controller topology?

[V_King]

Hi,
I am building myself a small PV solar setup to charge deep cycle batteries on my boat. After a while of trying to figure out Chinese charge controller specification intricacies I have a question in regards to positive ground solar MPPT charge controllers.   
The market is seem to be flooded with positive ground MPPT solar charge controllers and the same controllers with negative ground are usually charged more money. It got me wondering how much do those controllers differ? Is it just a matter or tying either positive or negative rail from PCB to the chassis or should I expect more fundamental differences to the PCB layout and design? Please don't forget we are talking about the Chinesse solar charge controller which is same model, just positive or negative ground model designation.
Thanks,