I dont have a dog in the fight of which one is better, but I will say that for a lake boat, I'd stay with the jetaway. It allows you to start your boat on the trailer and from what I've seen of popoffs, they are allways coming apart, falling off, require quite a bit of maintenance, and to trigger them you have to either do it manually with a button on the wheel or try to set it up from intake press. to do it automatically which is a pain in the arse. I see'em at the track blowing of at half track all the time. Now I know that the newer setups have addressed alot of these problems, but I dont know if they're perfected yet. And if you were to try to trigger it manually and had a violent engine failure I think you'd be in the water before you could ever even think about touching that button.
I did see a thread on here were a guy was trien to set it up so that when the jetaway would ratchet that it would trigger the popoff, thats seems like the way to do it. But this would still be a race only dealio.
There are a whole bunch of guys out this way (myself included) that are running over 120 with just a jetaway and I havent heard of any of them having any shutdown problems, and believe me some of these guys blow up there stuff on a regular basis. They lean on'em pretty hard.
My advice would be on your lake boat, to buy a jetaway on, runit up to 60 and cut the throttle and see how she feels, if OK then run it a little faster and try again, if it begins to feel like it was going to try to turn, or hoppity hop, then you need to try some hardware changes till your shut down gets nice and smooth.
Some of these guys crutch their boats with the popoffs and this allows them to run stupid-dangerous hardware on the bottom to try to lift their boats out of the water to run faster, and while it "might" get them a little bit of et of mph , they had better hope that that popoff and none of its associated hardware fails to open or they'll be going swimmin.
So if you rely on the popoff 100% to always operate, knowing full well that if it doesnt then you'll be in a crash situation, then the popoff isnt a saftey device at all, its a crutch that when fails will get you hurt. You would have been better off without it and had to tune your boat to shut down with just a jetaway.
Your boat should be able to shut down "HARD" with or without a popoff, then use the popoff for some added protection, if done this way you will have all the safety available with these two items.
That just my .02