In an ideal world I would fit both. I like the immediacy of knowing that I have charge flowing either in or out of the battery, but I also like having a voltmeter which gives a better indication of the health of the battery.
I have two digital voltmeters (one on each battery) and no ammeter though, as it wasnt really worth the additional wiring, complication and expense of adding one.
I did consider one of these, but decided against at the time;
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