We bought a new set, I think it is the Motorola 6200, but might have been 6300. It was the only Motorola with vibra alert. After much research, we decided the main features we wanted were channels and subchannels, AA batteries rather than AAA and vibra alert. Shopping for them, it became clear that Motorola was the premier name in FRS. We eventually bought them from Overstock.com because there was a coupon (my sister bought one and I bought the other, which gave us a coupon on both) available from Flamingoworld.com, I believe.
We are also owners of a cheap Jansen set that has channels and subchannels and have had good luck with them, other than the fact that they eat batteries (AAAs) and my dh lost the battery cover when he attached it to his belt. The Motorolas work fine with them, even tho we had been told that Motorola didn't work with all brands. There is a conversion chart in the user manual for brands that use different number configuration for their subchannel.
I was told that AA battery-type FRS have a longer range and longer battery life than those that use AAA batteries. The user manual says rechargable batters have about 1/3 the live of disposable ones.
I haven't used them at a theme park yet, but they work great in the neighborhood, through trees and buildings without a problem. They are relatively simple to use, even my 8 yo doesn't have any problem with them. The Jansen ones I had to carry the manual with me every time I used them.
Hope this helps.