Here are some thoughts I'll throw out there:
Working them in the back means they will get care sooner (defib, drugs, etc) and will have continuous effective CPR. Down side is you don't have as many hands to do things.
Working them in the ER means they will have more resources available to them (more staff) which theoretically would be of benefit however to get them to the resus bed requires rolling CPR which we all know is about as good as no CPR at all and you have to plan on 45-60 seconds MINIMUM from time of taking them out of your truck to on resus bed with proper CPR being done.