1) Z boards are "unlocked". Since they are prepared to sustain the heat that come with overlocking there they have higher heat tolerance.
I also found Z boards are generally $10, $20 more expensive than their H equivalent these days, maybe it is just a Canadian thing.
To overclock you would need a top tier power supply (think EVGA P2/T2) and a dedicated CPU cooler, which after all you would throw in another $50 and $60, respectively.
Then when you upgrade your i3-6100 to i7, you need to reapply thermal paste which is another $5, or $10.
By spending those extra $10, $20 to get a Z board these days, you are setting yourself up to spend another $120.
If you are not going to spend those $120 when you upgrade in 2 years, then why spend extra $10, $20 now...
2) My other concern is that you said you want to upgrade to i3-6100 because of the power consumption concern.
8350 consumes 125W while the i3-6100 consumes 60W or so.
What PSU are you using? My system consumes ~290W and I am running a 600W with it. I just don't want to run a cheap PSU at over 70% capacity.
Because if you PSU is already a 4 year old tier 3, then even you upgrade your system to i3-6100 with H170, your PSU fail and take a component or 2 with it, it is not really an "upgrade".