Mavericks or El Captain for old MacBook Pro (early'11) In case of performance for multithreaded applications, development (Java) and virtualization (Parallels 11), what's better - upgrading to 10.9 Mavericks or El Captain? (or Yosemite) ? 

Now I'm on 10.8.5, MacBook Pro 15", i7-2.0x4, on SSD with 16GB RAM

P.S. upgrading needed as Parallels 11 doesn't support 10.8
Go El Cap. The marginal performance decrease (especially on your machine) is near zero compared to 10.9. You'll get better support, newer features, newer apps, etc.