it appears that many tweed amps like the Bassman have a bright channel, and a normal channel.

but the 5E3 actually has an instrument channel, and a mic channel.

my questions are:

1) How do these two channels differ in tone?
2) In what other ways do these two channels differ?

in terms of sound they are much more different from each other than modern amps with a bright channel or switch.
The channels interact quite a lot, e.g. when using one of them, turning up the other volume control drastically changes the EQ curve.
On one channel, turning up the tone, the tone rolls off again after 3 oclock, whereas it works as normal on the other channel. these were early bugs in the fender design, which are enjoyed as such by most vintage gear lovers. You can fix these to more resemble modern amps, I found a few fixes when I searched to understand this. I've left mine as-is, why have all your amps work the same way?

see Topic: Volume controls are like toggle switches, to see how controls work

Choosing one of these 2 channels will vary with guitar, mood, day of week.
You can use both at once, but it adds some noise

Cheers
T