The nearest expiration date for WTI Midland (Argus) vs. WTI Financial Futures (Oct 2015) is Oct 30, 2015.
The best time to sell a futures contract is when you've already made money on the investment, but still have plenty of time left before the expiration date. Thus, a good option would be to sell WTI Midland (Argus) vs. WTI Financial Futures (Oct 2015) before Oct 30, 2015.