the meta gem can proc exactly when u want and get u good speed on a bolt or 2, or it procs when you make healthstones or summon a pet. The 12 crit is not alot but its still nice, bigger bolts more crits is always nice, and a 5% stun resist is just as rng as the haste meta. You can go a million years without it proccing, or it procs on a dr warstomp you didnt really care about, or it procs on a full kidney and saves your life and ends theirs.
both are rng that can save you or it wont do anything. But you can always count on 12 crit.