You should just weigh the differences in the two different engines.
You have to give a big weight savings advantage to the 400 small block, and also remember that parts will be quite a bit cheaper for the small block also.
The 396 is heavier duty, and will make more horsepower due to it's superior breathing ability, and it also lends itself to more cubic inches if you want to stroke it, or bore it more, where you are limited with your 400.
And, as several have mentioned, the originality of your car, but that is just a personal preference, that you must decide. You have to make yourself happy, not everyone else.