ability word with modifier, OR alternative keyword like devotion

"...where X is your commitment to Islands as you cast ~". If you're varying the number of targets.

Anyway. Yes, I could see this. The Jaws of Stone mechanic probably has a bit more design space.

Number of targets can't change after you cast anyway. however, damage can, that's why jaws of stone is worded like that.

Ah... the card is a question, not a modal spell. That took me a little while to parse.

My first thought was that the ability word is stronger... but, you know, all that does is lock you into a format that might not work if you need to use the word 'commitment' multiple times on a card (which shouldn't happen often, but could. For example, you could have a card that helps you for being committed to Forests, while it hinders your opponent for being committed to Swamps.)

Ability words work best when you know the mechanic is a one and done sort of thing. Like Landfall... it's in Zendikar, and it's gone, and we don't need to reference it, or work around it. It's just the simplest way to group an ability. For something evergreen, though, I think keywords are best. And this is evergreen. It probably shows up as often in the core sets as Regenerate does (M14 features Nightmare and Quag Sickness vs. Trollhide and Vampire Warlord). Whether it's wise to keyword every evergreen mechanic... that's a different discussion.

("Number of targets can't change after you cast" - precisely. Which is why this card would need to read "the number of Islands you control as you cast ~", or "your commitment to Islands as you cast ~".)