12AX7 - Original RCA design
12AX7EH - 12AX7 made by ElectroHarmonix
12AX7WB/7025 - 7025 is low hum audio tube, W means military
12AX7LP - Long Plate - marketing - basically a normal 12AX7
12AX7LPS - Long Plate Spiral Filament - due to hum problems with
normal LP
12AX7-C - Don't have any idea


What accounts for the difference?

Some, such as 7025 are legitimate differences in design, the rest are
basically the result of manufacturer's trying to make their product
stand out in a field of very similar products.

It also appears that
12AX7 is the same as ECC83... is there any benefit
switching to this type for microphone pre-amp?

ECC83 is a euro 12AX7, and you will find the same tubes with either or
both labels on them.

Your best bet for a mic preamp, is to buy 12AX7s of any type, which have
been selected for low noise operation, and low microphonics. The price
will be higher, but the results will be worth it.