I’m new to this, so I may be wrong (or I may have the perspective to answer depending on how you look at it), but the higher cash requirements for the ES, and the psychological difference present due to the underlying values. MES is pretty easy to see as relatively low risk. I may make a bad trade, I may compound a bad trade and wipe out my whole account in moments, etc - there are obviously endless ways to lose money - but the ES presents a much more obvious way: it’s just more money we’re talking about to begin with 
But, I had forgotten about the up-front cost of the commission-free pricing, and that’s definitely valid! Especially since I don’t even think that would be deductible at tax time as I understand it (I’m still trying to understand the taxes though for sure)!
So here’s my situation, both as an example and because I obviously need to think this through myself at some point so it might as well be now! I’m only bringing in $550 right now, so I plan to focus on the MES. If I filter my 3 days of demo data to just look at my MES trades I’m up $311.44 total P/L and it shows $268.56 commissions and fees on 746 contracts (I’m not really sure why that adds up to $0.36 commissions and fees, I’m assuming I either don’t understand something - which worries me - or it’s a weird fluke of being on demo?) Regardless, if I were to be on the membership-free account I’d presumably be looking at $0.25 more per contract, which would change my P/L to only $124.94. If I actively traded 15 days a month, and my small sample size was somehow fairly representative of the future, then I’d profit at the exact same rate for a month-end total of $624.70 profit. But, if instead I had committed for the year and ponied up the $2400 (so $200 for the month), then after deducting that $200 cost from my monthly profit just one time, I’d continue to go up at $311.44 after that, for a month-end total of $1357.20. Which, if I could even remotely consistently pull that off, is a good motivation to keep going for the whole year I had paid for!
But, alas, I don’t have $2400 laying around which is why I’m only funding for $550 in the first place. So I suspect I’ll either end up doing as you suggest and switching to the ES as soon as possible/comfortable, and probably paying the $100 on a no-commitment monthly basis to get Active-Trader since it would definitely be worth it still, or I’ll stay low risk & low reward and stick with MES until I really feel like I’ve proven myself and am willing to dump $2400 in to commit. You’re probably right - I’ll likely go with the ES. Guess I should build up my demo account more and practice with that then!
Edit: The commission-free plan can be had for $350/mo without an annual commitment. That’s almost definitely the way I’ll go as soon as I can build up $350 in the account or just fund it externally. Even at the higher cost it would still put me at double the profit based on my data so far.
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