with the 103RE it is easier/faster to "see", which side of the racing course is better for the run (in reason of wind shifts during the race). Each side you sail, you have a seperate scale (a red for the port side / a green for the starboard side) on the 103RE. With the 103R you have to think and count a bit more, because you have only one scale for both sides and so, you may are not with 105% of your concentration to the sailing.
I have seen, several pro-racing Laserites do not use the NEXUS (here in Europe we use the "Silva", that is the same like the NEXUS at NA) anymore. It´s their experience of "reading the wind" from the surface of the water (and fixed points at the shore) they sail in, that they do not need the Compass and can make their boat a bit lighter in weight.
The easier to use is the 103R, instead of using a 360 degree scale, it uses 0 -20. in one tack you are 9, on the other tack you are 19. No need to calculate the 180 / 90 degress difference for the other tack. If you read the instructions, it will tell you how to use for the reaches and downwind, in case where you sail you cannot see the marks due to longer courses.
Unless you are a top sailor who is going to write all the numbers on the deck, this compass is great.