General education. The right answer depends on your house.
Ask where you want the water
If you want it at the shower, the washing machine and every faucet, you're talking about the main line, so a whole-house system. If you mainly care about a glass of water and the pot on the stove, you're talking about one tap, so reverse osmosis under the sink.
They're different tools and they aren't competing grades of the same thing. A larger system isn't automatically a better one for what you want.
Then ask what your home allows
Your home often makes the decision for you.
- Condo or shared main line: usually reverse osmosis, subject to building rules.
- Main line in a garage or on an outside wall with space around it: whole-house is on the table.
- Full sink cabinet, no faucet hole: whole-house might be easier, oddly enough.
- Rented home: ask the owner before planning either one.
Doing both
Plenty of homes end up with both, a whole-house system for general use and reverse osmosis at the kitchen. There's no rule that you have to start with both. Starting with one and adding the other later is common, and it's a fair way to spread out the cost.
