Silver interconnect: XLR, or RCA Phono ?

Dear Linnies,

I have a balanced AK4200, and it's fed from my AKontrol/0 with a pair of 3m Silver interconnect. Terminated with RCA Phono for the pre-amp and in to XLR for the power.
I'd now like to use one channel for the centre speaker. Two questions for your considered opinion, please.
1 - Is it advisable to match all cable lengths (ie. the other pair at 3m)?
2 - I assumed the actual cable was the same, merely that the terminations are different. However, a retailer asks that I specify XLR cable, or RCA. Which should I select?

I have this Info doc, generated by Linn - but it doesn't specify if you should use XLR over RCA, if indeed there is a difference.
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