Face Down + Hidden Mana

Probably I attack past it while keeping one monster back for defense, saving the monster-D for next turn, especially as an opponent with that much mana has presumably been building this hand for a while and likely has a counterspell. But that's a strategy answer, not a design answer, and depends a lot on the gamestate.

Really the decisive factor between these two versions for me isn't flavor--though I admittedly prefer the limited information approach to the no information approach because I think working with limited information is more interesting*--it's that I really don't want a tedious mana-tracking mini-game added to CMC. Like, verging on "I hate clickwars"-level don't want.

Mind you, there are ways to try to get elements of your vision with the tiered approach as well--things like

RoboNPC
0D4G4L
zero
(Facedown)
When Decoy is destroyed, gain 2G2L and draw two cards.
1/1

and

Mobile Mine
4D4G0L
zero
(Facedown)
When targeted by a spell, deals 40 damage to that spell's controller.
30/50

leap to my mind as rough drafts

*This probably goes back to our differing opinions on randomness, actually. You tend to see randomness as a way of making the game more interesting, whereas I tend to see it as making the game less interesting after a certain point. And obviously we disagree on where that point is, and this discussion is unlikely to produce any major shifts in opinion from either of us on the matter. Which is why I'm trying to lean instead on the "mana-tracking is unfun" issue, which I hope we can agree on more readily. Since, y'know, it's your game and I'm just a rabid fan.