1997 bmw 3 series radio

That depends on the radio you're putting in there.  If it's a high end pioneer or a brand such as Eclipse with 4V preouts or higher then yes, you should use the one with RCA connectors.  70-8591, sticking with metra numbers.

If it's a lower end head unit or one with lower voltage on the preouts(it should be listed in the specs of the radio) then I'd recommend using a 4ch LOC from somewhere like soundgate or PAC and the regular 708590 harness.  This would allow you to get your output matched as close to the factory radio as possible.  Since there's no gain adjustment on the factory amp, you'll have to do your adjustment at the head unit end.