Unlike vintage cards where value is driven purely by condition scarcity, modern SIR value is heavily supported by the aesthetic demand for the specific artwork
Our newly launched Buyers Guide section is your first mate in charting these waters
Then you have the card's effect: in Gifts Ungiven, you search your own deck for cards, whereas in Gifts Given you search your opponent's deck for four cards
In the case of Affinity, having another free artifact may actually make you more explosive, so its definitely the card I would choose for my sideboard
Many Time Warp variants and Jace planeswalkers are present, which makes sense since those sorts of cards are often reserved for mythics
Centering and surface condition matter a lot here, especially with full-art SIRs that tend to show flaws more easily