Lot's more grunt with the 79cc Predator and the price is right. I've used both the 49cc hs and the 79cc and there's a big difference in power.
On the other hand the 49cc is narrower and if you have three speeds to work with the smaller engine should be powerful enough.
I have done both as well and agree.
The machines I am building these days are using the HS 144F 53cc engines and an internal 3-speed and find them not only easier but flat out awesome.
...She just said she wanted a 4 stroke with a centrifugal clutch and to be able to keep up with my friends and I.
I want her to be able to cruise between 35 and 40.
Those are exactly the bikes I have been turning towards building.
Heck, I don't even dink with 2-stroke builds unless they are extreme custom builds.
Sure they cost more, especially when equipped with dual disc brakes to be able to stop as fast as you you can go in 10' but will still run with modded 2-strokers in every way.
With 3-speed gears a 49-53cc shifter you just don't have the inherent hassle of a 2-stroke in every way short of weight, and I have spent the last 6 years trying almost every way there is to motorize a bicycle, and found what I think is best system there is for reliability and ease of operation for those that want a mini motorcycle to get around with high speed and minimal hassle, and going 'all in' with it.