1. Should I buy bitcoin with my $6000

Not a bad option but your ROI will be a finite amount based on the rise/fall of bitcoin alone.

2. Use it to buy a 1070 GPU mining rig

Mining offers a longer and higher ROI than direct investing when using some general assumptions. As far as a 1070 rig goes, there are many better options out there based on upfront cost + energy consumption to hash rate.

3. Wait until July for new nvidia graphics card to come out (or the asicminer equihash).

This may be the best option as far as the new GPU's however you risk losing out on gains in the meantime especially as availability for the new cards will be scarce.