I think the long answers to both of these in Codex are almost always “it depends on a lot of factors and each situation is fairly unique”, those factors of uniqueness being the matchup, the current turn, the current board state, your current deck cycling state & gold (aka econ state) etc.
I’ll try to boil down basic heuristics I’ve used when starting out to answer each individually though:
###What considerations should be made when determining whether to play a card you can afford to play vs. not play it and draw an extra card next turn?
Usually this boils down to “if I play an extra card/two NOW, what is the risk I get that board advantage erased, and what are the chances it leads to a big play this turn or next turn?”
Things to consider when wondering “risk of getting wiped” are:
- How much haste / direct damage / direct removal spells does my opponent have access to
- How likely were they to tech those cards, given the events of the game to this point
- How likely are they to have those cards in hand, given their current cycling state
For example:
- playing against Anarchy, Zane could come out and bring sharks, that’s 8 hasty damage across 3 attackers for 7 gold.
- Strength tech 2 could drop Degrey and erase all tokens, dangerous against a Feral frog rush or Necro skeleton army.
- Law could bring Bigby + Judgement day to totally wipe tech 2 units and below.
- Necro + black starter has Doom Grasp for your big baddy, sacrifice the weak for your lowest thing, deteriorate to finish anything on 1 health…
You get the idea
To answer “will this make a big play this or next turn”, ask yourself:
- Can I take away what I’ve determined is my opponent’s “primary win condition” (a maxed out hero, a strong tech 2 deck, a tech 3 unit, etc)
- Can I set my opponent significantly back on board position that will lead me to locking them out of their primary win condition (killing a tech 1 when their hand is likely to have one, take a hero away when their hand is likely to have a key spell)
- Can I put enough damage on base that I finish it or leave it very near finishing, and will I win a race to finish it (this only applies to certain decks, primarily decks with any Red, Demonz, Peace, or Bashing, that have strong access to direct base damage)
When is it appropriate to tech two of one card vs. one each of two different cards?
This is more about “reading the opponent”. If you are very certain of your deck’s win condition and what your opponent is doing, and you’ll be cycling soon that you want the best possible draw odds, it is absolutely appropriate to tech 2x of the same card.
One thing you’ll find about codex that’s different from magic though: your opponent has many reasonable options, you’re often unsure what they’re up to, and teching 2x of one card may spell death for you if you guess wrong on which single card to tech 2 copies of. Also, you are almost certain to see whatever you tech in the next 1-3 turns, and with heroes, tech buildings, and add ons always being available expenses, you often want to spend your gold on something OTHER than what you teched anyway. Even if you tech 2x of something, you likely will only have enough gold to play 1 of it when you draw them.
Therefore, it is often wiser & safer to tech 1 of a thing that’ll help against 1 route your opponent might go, and 1 of another thing that would help against a different route. Or 1 expensive thing, and 1 cheaper thing, in case you want to invest in a hero / tech building instead (there isn’t enough gold turn 3/4 to play Rambasa twins AND build tech 2 very easily, so as white I’ll often tech Rambasa Twins AND Martial Mastery or Sparring Partner, so I can decide between teching up, beefing a hero, or the tigers, and always have use for the gold).
I hope that this helps, and I’d encourage you to play some games via the forum; it isn’t the same as playing in person, but it will help a TON in getting you familiar with matchups and how to make these kinds of decisions.
Welcome to the light, I hope you enjoy this game!