I can't conceive of any downsides, only benefits. I'd recommend a 16mm phbg if you're using the 50cc stock cylinder. The phbg has multi-circuit, which in laymens terms means you can adjust your fuel to air mixture for different throttle positions... whereas with an SHA you've got one, single jet.
If I could do it all over again I'd have gotten the phbg right from the get go---- cuz now that i've kitted I had to buy one and it ends up costing me an extra $100 in the long run. If it's stock, you likely only be tuning the main jet anyways so I don't expect the learning curve to be steep. But if you dive in, your stock cylinder might be more responsive cause you can really get in and tune for best acceleration.
I'm just learning to tune a phbg now, and using the wiki and moped tuners spreadsheet looks like I won't be tuning too many things even with a kit. Most people seem to run with the stock 50 pilot jet and w7 needle at position #2.
go for the gold duder... get a 16mm phbg and larger intake, should be big enough for those 70cc parmas benji's getting in soon :)