I'm quite confident that raising the Trapinch with Arena Trap would do you well in the long run. As you've identified, it's a much better ability for player v. player use, and choosing it shouldn't disadvantage you too much in the short-term either. It's more about where you are in the game and how soon you can correct the mistake.
You indicated that you have trained a Pokemon to level 30 'so far' -- how high can you actually go at the moment? Trapinch can learn Dig at level 41 if you hold back its evolution, so the Trapinch with Arena Trap won't have to go long without Dig. Also, if you can access the Seafloor Cavern, there is an Earthquake TM there which makes for a much better move than Dig anyway.
If you're at the point where you're training level 30 Pokemon then you'll probably be off to the Seafloor Cavern soon, so I'd be picking up the Earthquake TM there and teaching it to the Trapinch with Arena Trap. I don't think Dig is worth the fuss; you'll only have it for a while before you replace it with Earthquake.