Alex - thanks for the videos, which are a great comparison.
There's no shortage of differences between the original and new FEF3s in terms of operating features. I have the 844 Greyhound from the first issue and the new 8444 and the three differences that matter to me are:
1. The sound set: I agree the new version main sound is different and better; to my ear it sounds crisper and deeper with the addition of some extra sounds. However the crew talk is no longer engine-specific. I know that won't matter to a lot of people but I'm a little puzzled by it in the light of there being one extra segment of dialog as described in 2.
2. Lighting: On the original version the Mars hazard light flashes when the engine is stationary, including when it is simply "parked" on a powered track. In the new version it only functions when the emergency stop sequence is engaged and there is new tower/crew dialog to go with that. The new version of this feature is more prototypical in that the hazard light on the real engine was seldom used, or so I understand from some research. There's a thread on this forum a couple of years back that discusses this.
3. Smoke: The new version has a two chamber smoke unit to run the main stacks and the whistle steam; the original has a single "analog" smoke unit with a 6 or 8 Ohm resistor (I forget which it is) that Lionel no longer uses or stocks as a replacement part. The original unit is one of the most prolific smokers Lionel has ever made and puts out plumes of smoke and indeed the occasional smoke ring. The new unit makes a fair amount of smoke but nothing truly exceptional by either Legacy or MTH standards. It does however have smoke lofting out of the stack at idle, which the original unit did not.
Finally, with the new Legacy control boards and using command control the sound, smoke and light functions are all off (apart from the marker lights) when power is applied to the track and until the engine is addressed. In the original there is power to the lights and smoke unit whenever the track is powered even if the engine has not been addressed. It also has the distinct background "buzz" of the earlier Legacy boards whereas the new version does not.
I actually like running the two engines at the same time.