sediment and carbon block? I think all the filters are the same, the only thing that sets them apart is the RO membrane. The first and second just get it cheap.
If your first filter is a sediment cartridge, decide before you order what pore size (measured in microns) you want. 5 micron and 1 micron are common in this hobby.
Is your next filter a carbon block or GAC? What is the pore size you'd like on the carbon block? All carbon blocks are not created equal! Check the chlorine capacity before you buy.