Here's what I've learned:
Crimping is superior, but who says you can't do both? For my off-road car I crimped every end connector and spot-soldered the ends. For butt splices, I typically don't solder.
I buy my connectors from these guys:
http://www.pelec.com/
I use standard PC-type M/F connectors (You can get them as high as 20pin). They're cheap; you can buy new pins for the connector in almost any size for pennies.
Note: Get the LOCKING kind
I use the crimp tool they sell for $15. I strip the ends off the wire, crimp the pin on, spot solder the pin to the wire, slide the pin into the connector and use a syringe to fill the cavity with black silicone. When adequately prepared I can do a wire a minute.
The great thing about these is once you've connected them you can zip-tie them together. I assembled a complete harness for my rail with these that comes apart in three pieces, so the motor comes off with no wiring headaches. The car has been through hell and high-water and it has not failed yet!