In the case of Urza, having him along with The Mightstone and Weakstone in play costs 8 mana total, not to mention the additional 7 mana to activate Urzas melding ability
While they are worse than Nature's Claim, when youre down to one mana, having limited removal in hand beats having none and if your Natures Claim is already in your graveyard, they probably wont see these coming
SIRs appear ~twice as often as normal (~1 in 45 packs) but with 32 SIRs in the pool, each specific SIR is ~1 in 1,440 packs
However, it's the second ability that really stands out
Of the two starters, its probably the winner but requires a bit more planning and thought for the best result
Aang's Iceberg Aang, Swift Savior Aboleth Spawn Accelerated Evolution Activated Sleeper Adaptive Shimmerer Ademi of the Silkchutes Aether Meltdown Affa Guard Hound Agent Venom Airtight Alibi Akoum Alchemist's Refuge Alexi's Cloak Alms Collector Aluren Ambrosia Whiteheart Ambush Gigapede Ambush Paratrooper Ambush Viper Ambush Wolf Amphibian Downpour Anafenza, Unyielding Lineage Ancient Stone Idol Angel of Grace Angel of Salvation Angel of the Dire Hour Animate Spell (playtest) Annie Flash, the Veteran Appa, Steadfast Guardian Appendage Amalgam Aquitect's Defenses Archangel Avacyn Archivist of Oghma Arlinn, the Pack's Hope Armaggon, Future Shark Armor of Thorns Ashcoat Bear Ashiok's Erasure Ashiok's Forerunner Asinine Antics Aspect of Manticore Aven Interrupter Aven Mindcensor Aven Reedstalker Avengers Assemble