The short answer: join OnePass if you'll be flying CO metal.

The long answer:

When it comes to picking a SkyTeam program, it's usually best to choose the program whose metal you actually fly the most, for the following reasons:

• The elite phone number is only available on your "home" carrier. DL and NW Plats still have to use the plebeian phone line when calling CO.
• When it comes to irregular ops, home team elites will get priority over partner elites (but partner elites do get priority over base members).
• Many mileage bonuses (online purchase/checkin, promos, etc.) are valid only on home team metal.
• Free domestic upgrades are generally only available on home team metal (except for NW/CO, which give each other's elites upgrade reciprocity). Same goes for mileage upgrades.

That said, if you're flying mostly internationally (presumably in paid business class), you'll probably find the benefits of accruing miles to DL vs. CO to be only slightly in CO's favor. Cheap coach fares on each carrier will only earn 50% EQMs in the "other" carrier's program. But regular coach fares earn 100% EQMs either way, and full coach and paid F/J earn 150% in either program.

CO elites earn slightly higher elite banked mile bonuses on CO flights than DL elites do (CO silver/gold/plat earns 50/100/125% mileage bonuses, while DL earns 25/100/100%).

But DL miles are probably still worth a little more per mile due to less restricted award availability.