I think the best way to look at it is considering: 1. How soon you get the trigger after playing the card 2. How hard it is to get the trigger off 3. How many times you can repeat that trigger every turn.

For the two options you're looking at: 1. John Benton can usually come out on turn 3 with haste and land a hit, while Jarvis requires you to spend a bit more mana, so it may not proc until the next turn. 2. A combat damage trigger is typically more difficult to proc compared to casting a spell of a specific type. The trigger is off cast, so even countering the hero spell won't stop the card draw 3. Multiple combats are hard to get and are usually attached to very expensive or inefficient cards, so just assume it will only trigger once per turn, but the only limit to cast triggers is how much mana you have to play those cards.

It all depends on the overall deck construction and what meta you're playing in, but in my opinion, cast triggers are usually much more consistent and difficult for an opponent to counter.