The market is full of water filtration systems, and is best to investigate on a system that you can get replacement cartridge locally.
Price & quality wise is vastly different between models & manufactures, and some inexpensive systems are just as good as the more expensive ones.
I wouldn't be too concern with the volume gallon rating on the filter cartridge, because water sediments/conditions vary from place to place. Larger filters will last longer and tend to deliver more gallon/minute but for most home use any smaller system will do.
It is best to have your water tested before making the purchase because your local water may be good enough to go with a cheap/low up keep 2 or 3 stage filter system.
2 stage tend to have a multi grade filter 5/30, 5/50 or 5/75 micron polly filter as the first stage then a second stage 5 micron or less carbon filter.
3 state is similar to the 2 stage but the sediment & pre carbon filter is separate filters unlike 2 stage multi-grade system (3 stage tend to have longer service life span than 2 stage).
RO is an over kill for most city water system, however it is more effective at taking out minerals/bacteria/germs/parasites than 2-3 stage filters. RO system cost about 2X more than 2-3 stage system and up keep cost is slightly higher as well. That said, RO is the way to go if you have the money for it and want to remove unpredictable quality of city water.
RO is great to use with well water system combine with a coarse sediment or bio-sand filter, and a UV between the sediment & RO unit would make your water that much more kosher.
Good luck!