PHP on Windows Upgrade



I currently have a Windows 2003 server running both IIS (for classic ASP support) and PHP version 5.2.6

I have a new server, Windows 2012R2 again running IIS, and I'm trying to figure out what to install for PHP...

Should I:

1) Install 5.6.21 or should I jump to 7.0.6 - and if I go to 7, will all my PHP code still run ok?

2) What is the difference between "Thread Safe" and "Non Thread Safe" - this seems to be an important choice when downloading the correct version, and I don't understand what the difference is....

Thanks for any information!
The differences between TS and NTS: