Mystical summoning is nearly identical to using a Mystical Scroll to summon, with pool of monsters available and the summoning odds being completely identical. The only difference is the cost of summoning, as Mystical Summoning requires the payment of 75 Crystals instead of the 10,000 Mana Stones used for Mystical Scrolls.
While the crystal to summon ratio is basically the same as buying Mystical Scrolls from the shop through the Summoner Pack and Premium Pack, both packs also contain Angelmon as bonus items in addition to the scrolls, making the packs a better value than Mystical Summoning using crystals.