What's less expensive, filter-wise, filtering only the water you 'need' or
filtering all the cold water out of the tap?

I don't know for sure, but I spend $30 a year and never worry about putting
the filter in the right position. Those cartridges for the smaller units
are not very cheap.
---------------
If Ed's install is similar to mine (OmniFilter), there's a separate
spout mounted on the sink deck for filtered water. Only the water I
want to filter gets filtered. I go through 2 filter changes a year,
which costs about 45.00 and is VERY good filtration.