FaunaDB's query language makes it straightforward to do it the first way. All queries are serializable, so any preconditions checked would gate transaction commit as you would expect, and read-modify-write style transactions work.

edit: here's an example in our Scala DSL:

  Let {
    val amount = 50
    val balanceA = Select("data" / "balance", Get(Ref("accountA"))
    val balanceB = Select("data" / "balance", Get(Ref("accountB"))
    If(Gteq(balanceA, amount),
      Do(
        Update(Ref("accountA"), Obj("data" -> Obj("balance" -> Subtract(balanceA, amount)))),
        Update(Ref("accountB"), Obj("data" -> Obj("balance" -> Add(balanceB, amount)))),
        "Transfer Success"
      ),
      "Insufficient Funds"
    )
  }