The 717 is cheaper to fly on a per departure basis than a 737, the cost crossover point is around 1100 miles. So I would imagine you will see, after some fence, the 717s moved strategically around our system to take advantage of the lower cost per departure and the seat count.
In regards to SLI, I don't think any one thing: seat, relative sen, DOH, career expectations, career earning, final career seniority...etc. will be the only thing used. It will be a mix of all of these. Which seat you are in the day the deal is done will probably be the same seat you are currently sitting in today, but after the fence agreements expire, the "global" seniority list takes over and you sit in the seat at which your seniority can hold. imo
I also believe a "windfall" in regards to monthly pay, qol, schedule, career earnings, retirements, benefits, flexibility... goes along way to convincing an arbitrator that which side of the aircraft you sit on is irrelevant. Except to massage some peoples egos.