3 card would be most beneficial in a Grixis spellslinger deck, as it rewards you for playing noncreature spells with its ability to deal damage to each opponent
A 1/1 elf with infect for one mana, a turn one Glistener Elf is often an ideal start for a deck seeking to win through the use of poison counters
If you're chasing the most expensive cards, you might consider the oldest sets (as described above) the best, as they give you a chance at pulling something worth new car money
Since you can play it during another players turn and at instant speed, you can catch them by surprise
Almost every cube has shock, sac and original duals
In terms of their playability, both decks are, in my opinion, recommended for tactical-enthusiastic players