Hi!
First, Mini-30 is 7.62x39, AK caliber, not 7.62x51(?)/.308, M1A caliber.
That said.. between those two choices, I'd say M1 Carbine, for the mag issue if no other reason. Make it a Mini 14 instead of .30, and you're a little closer to a toss-up as to what's most important to you.
Finally, personally, I don't think an inaccurate rifle is a "challenge" but rather a frustration. A challenge is something that you, through your actions, can meet and and conquer. But if the GUN is so inaccurate Carlos Hathcock himself couldn't hit a baseball at 100 yards with it, then it's no longer a question of skill, just patience with frustration...
.. unless you're approaching this from a gunsmithing standpoint, rather than a shooting standpoint, in which case it's quite right to call it a challenge.
not meaning to criticise or anthing.. hoping to help..
-K