First off, let's compare it to our only alternative Dialga, from DP/GE.
Dialga MD has a few advantages stats-wise over it's previous version(s). 10 more HP, and the use of Adament Orb (weakness does 20+ more) add a few perks, but that alone isn't much. Also, keep in mind that not much is weak to :metal:. You won't get to use Adament Orb often. :frown:
But that extra :colorless in the retreat cost is sure going to be a pain...
Both have weakness of :fire:+20 (let's face it, all the big fire attacks would KO it anyways), and a resistance of
sychic:-20 is always good against Gardy.
Now attackwise, they're also relativly equal.
Time Shift and Time Bellow both have similar effects. Draw a card and do 10 damage, and draw untill you have 6 cards. Time Shift (MD version) is probably better in most cases, but not every deck has a very small hand for long.
Again, we see how attacks have different uses. Diamond Blast needs one more energy, but does either 20 or 40 more than Flash Cannon. But removing energy with Flash Cannon will de-evolve, doing the equivlent of 20 to 40 more damage, and the Defending Pokemon loses it's best attacks.
Overall, I'd say run one of each. But why run a Pokemon that is kindof lousy? Well... there's always Dialga Lv.X...