Merlins ro pros : cheaper initial outlay, built in pressure switch combined with float valve automatic shut off,compact design, takes same prefilters. Cons: needs ideal pressures of 80 psi may need to be booster pump,higher running costs - resin, waste ratio if you on meter. Ppm in hardwater area 350 up, 20 upwards after membrane. Slower production rate but a lot faster than your current setup.
4040 higher initial outlay, bulky, more economical to run, less frequent resin changes, you will get away without a pump on low pressure version. 300 litres an hours plus achievable if pumped. 120-180l/hr non boosted rates achievable depending on pressure and temperature.
Both need resin vessels 11lts min not to mess around too often with changes.
Choice is down to personal preference, ppm, funds and space available.
I have boosted 4040 with twin resin vessels and 20inch prefilter on auto shut off valve.