First Valley of Mines or Jharkendar?
 

I'm still in chapter one, I have done nearly all quests which can be done before choosing a guild and I am busy with the broken key pieces quest for the water mages.
I know I can't get the piece from Lord Hagen before joining a guild.

However after I got it should I first visit the valley of Mines or Jharkendar?

That depends on how you wish to progress, as both options are viable. Personally, I let my class determine where I go first:

- Mage: Go into Valley of Mines first, do the quests there by (mostly) running. Get into chapter 3 as soon as possible to get higher level spells before doing Jharkendar.
- Mercenary or Militia: Jharkendar first in order to get a few more levels and a certain sword so Valley of Mines becomes easier.