Battery connections!
I've done many power connections for Ham/CB/Audio applications.
You are always better off, from a noise standpoint, to make direct battery connections.
The two big batteries we have in our trucks (in parallel), act as one big battery, and it does not make much difference which one you connect to. Ease of connection should be your primary concern.
There might be a very small noise level reduction (recieving) by connecting to the battery not directly connected to the alternator which can be a small noise factor.
Direct connections are the way to go.
Rog