Since CO flies to BJX but NW doesn't, and since AM flies BJX-IAH but not BJX to any NW hub, that would lead me to think you're more likely to take CO flights than NW flights, which puts a vote in favor of crediting your miles to CO.

But if you're travelling from LAX and ATL (neither of which are CO or NW hubs) to other places in the US, then NW may make more sense, because

1. NW gives 100% elite qualifying miles for all AM and NW flights (50% on cheap CO and DL fares), while CO only gives 50% for the cheap fares on any of the four airlines (except CO flights purchased on continental.com). If you don't fly on cheap fares, this isn't really an issue, but if you do fly on cheap fares, choosing NW will make it much easier for you to make silver (or even higher) elite.
2. there are more first class seats on NW planes than on CO planes, so your upgrade chances as a silver are much better (silvers have a tough time on CO)

All of that said, keep in mind that elites from either NW or CO are entitled to free upgrades on both carriers.

Unless the 50% EQM thing is a big deal for you, I would probably recommend going with the carrier you're most likely to fly most often, because silver elites from the "home" program are presumably given preference for upgrades over same-tier elites from the "other" program, and another advantage of elite status is access to the elite telephone line, which is only available on the carrier where you have status (NW plats have to call the regular CO line, and vice versa).