The Pro4 is the safe choise, since it's a proven car, while the JRX-S aint avaible to the public yet.
One thing that really matter, is local support. Part support is one thing. Check out how well Losi and HPI is represented near you. ANother thing is support at the track. If there's many Pro4 cars, you'll be able to get a lot of help. That dont work for the JRX-S - yet. If there's a lot of Losi-guys at the local track, it might be worth waiting for the JRX-S.
Personally, I would go Losi, much more exciting design.