I go by my choice of products served: CocaCola, Bombay Sapphire and Cointreau. CO uses Bombay Sapphire fleetwide while NW only on international biz. NW serves Pepsi products (I only like Mt Dew) while CO uses CocaCola. Good thing is that I believe NW will switch to Coke b'cos of DL's position in ATL. I like drowning my ice-cream in Cointreau (only on NW int'l biz) or will switch to cheap port (usually Fonseca 27) on CO flights.

Go with NW if you travel more internationally. Once the DL merger is completed, you'll get 2 SWU (system wide upgrades) upon reaching platinum elite level. With NW, the gate agents in NRT usually will process op-ups more liberally when they see a platinum or gold elite cards. My op-up chances are about 80% out of NRT (mostly to HNL and SIN).

Go with CO if you will do most flights domestically in the USA. The EUA on short hauls from EWR and most flights out of LGA/JFK are usually good. Long hauls out of EWR (esp transcon flights) are usually battleground upgrades for gold elites.