It depends on the usage. For coyote hunting for instance, 98% of the time is spent behind the scanner. I prefer the resolution there. However, this again goes towards what it will be used for and the type of terrain. If all other specs are the same, a 640 thermal will have a wider FOV and less base magnification in comparison to a 384. An example of this is a Pulsar XP50 and XQ50. Speaking of coyotes again, this lends itself to having a wider fov and less magnification (640) for the scanner vs the scope. This can be different if hunting larger groups of hogs, etc. Speak to a reputable dealer who can help you through the pros and cons of resolutions, focal length, brands, etc.
I help people everyday sort through this type of stuff. Feel free to send me a PM if you wish to dig into this further.