Blonde or Brunette?
Seriously, it depends which one you like using. If you enjoy using both, get rid of something else. (There are at least 3 other pistols in your sig line I'd not ever buy, but that's me, and people like different things.
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I happen to think the 3913 (and variants) are among the very best TDA 9's S&W ever produced. I'd put one in good condition alongside any P239 or P225 and consider it an even match. (I say this as an armorer for both S&W and Sig, and having shot a fair number of Sig's over the years ... but I own a 3913.)
If I were going to own a surplus P6, and I wasn't an armorer, I'd have it inspected by Sig, probably having them do their Sig Service Package (SSP) on it. You never know how well a surplus gov pistol was used, maintained and stored. Ditto the magazines.
Then, as long as it consistently & reliably fed & fired any of the assorted hollowpoints I used, I'd keep it and be happy.
Bottom line? If I owned a properly serviced P6 surplus gun in good condition, AND a 3913 in good condition ... I'd keep BOTH. Neither are going to be any easier to find in coming years.
Suit yourself, though.