Most notable is the pre-amp designs. They are similar in performance but not identical. The original OctoPre is a typical Class A Platinum design, (based around a 737 Rhom transistor) the LE is an op-amp design closer to the idea of the original FF Green series. Both are solid state, have extended bandwidth (up to at least 180kHz at the top end) and sound transparent, but there is a subtle character difference; the original OctoPre has the edge in being more forgiving if you want to slam the pre's hard (but more 2nd order harmonic distortion as a result). The LE op-amp runs in a Class A mode (though is not true Class A) and is super-clean, but if you slam it hard will sound more transparent, (less 2nd order overall so less forgiving if you push it.) Hence if super-clean appeals LE has the edge, but if you like to drive a lot of level and are not obsessed with total linearity the transistor-based OctoPre is the way to go.
Aside from the pre design, the LE does not have a dynamics (compressor/limiter), control on each channel and only supports 44.1 and 48KHz sample via ADAT Lightpipe. The Octopre supports 44.1-96KHz and, depending on the ADC card you fit in it, can output in AES, S/PDIF and ADAT Lightpipe format. Other differences:
-The Octopre has 48V phantom power switches on each channel whereas 48V phantom is globally switched in and out on the LE.
-The Octorpe LE has a moving coil peak level meter which can be assigned to any channel.
-The Octopre supports 16, 20 and 24 bit conversion whereas the LE is fixed at 24 bit.
-The Octopre has line in and out connections on 25 pin D-sub connectors whereas the Octorpe has them on balanced 1/4" jack sockets (both have XLR inputs for mics)
-The LE has a word clock out connector as well as word clock in - the Octopre only has word clock in.
-The LE has an ADAT in port which allows you to convert ADAT signals to analogue and route them through the line outs (you can do this at the same time as using the mic pres and routing them to the ADAT out port).
If deciding between the two models for purchase, one thing you may wish to consider is that the Octopre will give you an extra eight inputs to your system but because the Octopre LE has D/A conversion it will give you an extra eight inputs and outputs simultaneously. You may want the extra outputs to route signals to a mixing desk.