"If you do manage to get the JAA PPL done out there the standard isn't as good, a bad thing if you're looking to 'go all the way' as it'll give you bad habits."
The standards are the same as in the UK. What is different is that it's not in the UK. You can talk about all the difference in the UK for RT, airspace, and such, but its not the same as flying in that environment.
However, being someone who has flown in both systems, they have high standards. They just emphasize different things.
Steep turns are steep turns. Landings are landings. Slow flight is slow flight. Both are taught almost exactly the same.
The differences come in stalls and nav. FAA treat stalls as if you will be stupid and eventually stall the plane and teach you to recover from there. The JAA teaches you to avoid it and recover before it happens. Nav in the US is taught by using pilotage, dead reckoning, and radio nav. UK nav really emphasizes the dead reckoning.
Learning to fly in the US won't necessarily give you bad habits, but there will be a learning curve transitioning when you get back home.