Like Gordy says, if you want a hope for an instructor job in the US, you will have to get the IR and become a CFII. Today a CFI will probably not do it alone ..
If you have no plans on flying turbines, why spend money on a transition ?? - you can't use that transition in JAA anyway. And if you do the JAA IR later on, you will get turbine training/flying there anyway ..
How do you know now what you are going to fly when you get your JAA certificates? If you don't already have a job in hand prior training, there is no way to know. So, get the certificates first, then worry about what happens after that until later on ..
Now, if you plan on getting the JAA IR later on (they do need a lot of pilots offshore back here), I found the knowledge very usefull that I achieved on my FAA IR (got it at HAI) when I trained for my JAA IR. It's cheap compared to the amount of training you get and gave me a good balast prior entering a more complexed aircraft like the Longranger - the step was smaller ..
There are a few differences between FAA and JAA, but not to much ..
- madman