My personal opinion is if your primary mission is trails/enduros and need the street legality...go for the DRZ400E. It is a better off road platform to begin with. Throw a dual sport kit on it and go.
To make the DRZ400S into a good off road mount it takes quite a bit as you really need to replace the CV carb with the e model carb (or other), get a plastic tank, and the suspension needs to be tweaked also. It is also a good bit heavier than an e model with a dual sport kit on it. The death wing tires on the "S" need to go immediately as they are neither good on or off road. They suck equally at either mission and will hurt you at the first opportunity.
Honest analysis will lead you to the right model choice as the "S" is the better base for 85-90 % road. As you go below the 85% on-road mark the "E" model gets more and more attractive. Quite frankly, if you want a 90% + street DRZ get the DRZ400SM and just get another set of wheels for off road.
For clarification (some will disagree), IMHO the "S" has a very limited niche market and does not excel at either road or off road. The DRZ400E is the off road/dual sport choice and the DRZ400SM is by far the preferred on road choice. Somewhere in between is the market for the "S".
my $.02