Agree with all of the above, but would ask you to think about this: what are your long-term objectives?
The skills of blocking balls in the dirt or framing pitches can be learned. A player can work to strengthen his throwing arm, but to some extent a strong arm is a genetic gift.
I would tend to start the boy who can block/receive better, but would look at the gifted arm as the long-term prospect. If he is willing to put in the work to learn his position, he has the greater upside, long term.