If you record the camera's playback to Ninja (or some other HDMI recorder), you uncompress the footage in-camera, resize it from 1440*1080 to 1920*1080 in-camera, then record it to your format of choice - which is usually a compressed one. This introduces an unnecessary compression cycle, which means you lose quality, at least in theory. Also, the scaling of the image is done in real time in the hardware, which is possibly not optimized for quality but rather for performance. You are likely to get better results if you do the scaling on the computer.
If you can avoid using an intermediate codec, avoid it. It's an unnecessary lossy compression step. Most decent NLE:s should be able to handle HDV as is just fine. Use an intermediate ONLY if you get a poor performance when editing in native format.
This said, devices like the Ninja have their place - if you record the LIVE camera output to the device, you can indeed bypass the in-camera compression. But as said thet does not apply to footage that's already on tape.