Selling old or new ESPP shares, which one is best?

[Ben Jonson]

Hey All, 

I have a stock plan I have invested some money in since 2015.

The stock went up and I have nice gains. My goal is to sell some of the stocks to balance my portfolio.

Considering Future Value, Present Value and etc, what period would be the best to sell ? 

(Capital gains tax are identical) 

Stock price: $150

2016:

Paid: $5000 for 100 shares, valued now $15,000

2018:

Paid: $8000 for 100 shares, valued now $15,000

Assuming I only want to sell one of these positions, obviously selling the 2016 position will give me more profit (even after taxes).

But is that the right choice ? if the market crashes, and I had money in the market which is better to keep in the market?

• 100 shares I paid $50/share for. 4 years ago, or
• 100 shares I paid $80/share for. 2 years ago.