I have a 1998 ExMark metro 48" walkbehind with 14hp Kawasaki. It is a tank. Very reliable, cut quality is great. Also have a single wheel stand-on sulky which is better on my back than sitting on a mower for hours.
Great point on the back pain! If you have a bad lower back a rider is pure agony on anything but a golf course. I have 3 riders as well but just use them for putting around the yard doing chores with a trailer or toting the kids around for fun. I absolutely loathe mowing with them as my low back is shot. A walk behind with a silky lets your knees flex and absorb lots of the punishment.