Where do you want to be in the next year or so ?
Ultimately it's the 4040 route - quick, high volume, brings my tds down from mid 300's to 004
I loved my Merlin, it served me well, I only sold it after 4 years because a 4040 system was in the van I brought, down side is water usage is slightly higher, but I boosted mine, and to be honest I never noticed a drop in usage when going 4040.
You can now get 2020's but have no knowledge of production rates etc...
Darran