OK, here's even harder maths. If you're doing 12,000 Round Turns per year in Nymex (your OP implies that you are trading Crude Oil Calendar Spreads) then leasing a Nymex seat for $500 per month and paying member rates would be CONSIDERABLY more cost effective than worrying about your execution platform.
I'm banging on this hard because over the years I've seen traders have so many issues trying to execute spreads using free platforms and discount brokers. What works OK for day trading ES does not necessarily make it so for a spread - even an exchange supported one. And the discount broker will almost certainly not have the risk system set up to margin spreads correctly.