I prefer racing in Masters than Open. Unless you get a good start in Open I always found the first lap a procession. With the Masters I tend to spend the first lap just hanging on! This year I have been racing the Gorricks in Sport, partly because the late start time has suited family stuff better (wife races Expert at the same time), but I also ‘enjoy’ the extra lap.
The pace at the front of Open and Masters tends to be similar, but Open does have a much bigger spread of riders. I would imagine the early start time of Masters, and that maybe only 30% of the Open field are the right age is probably why more don’t enter Masters.