id imagine an all strikes game is way easier than an all spares game, at least in the sense of not trying for an all spares game.
now if we're going strictly for a player attempting an all spare game as opposed to an all strike game, its definitely all spares. i'm sure enough people can strategically place enough balls into a small enough area to leave easy spare opportunities.
Now, if we're talking about actually trying to bowl the highest game possible. 300 is definitely easier than an all spare game. sure 12 strikes in a row isn't easy persay, but anyone accurate enough to make 10 spares is going to mess around and throw a strike at some point in the game