Use the ECM membership as it is more cost effective and the due dilligence on the part of the exchange is not as "probing" since you are not really a member. To lease a seat (or purchase) you would need to go thru the whole review process and be sponsored.
On the above link, IB has a link to the application. ECM is cheaper than membership, but the exchange fees are a little higher than if you were a member. The only requirement is that you avg 50 rt a day in volume to get the discount. Once you have this ECM, you are not tied to IB, you could use it for other brokers as well and I believe pool the daily volume requirement. Aslo, if you look at the unbundled breakdown on IB's commission schedules, for the ES, which you trade, the exchange fee is the same whether you lease a seat for a few k a month, buy a seat for, what 7 figures?, or use ECM plan.
But understand, the ECM is for trading your own proprietary funds, not anyone else's.