Depends on your space. On second thought, I guess I should say that since you mention either method, then you already have the space for it.

Aesthetically, I would prefer the blocks of 3x5 plants. Watering and taking care of them can be done in a smaller area. For future or other plants, this kind of rectangular design that you have already done for these plants may also be useful to the gardener.

Companion planting (if you want to do it) for the 3x5 area would also be more efficient to cover several plants together.

But for 2 long rows of 15 each, on the other hand, you could set up a kind of support system that can handle all 15 peppers and 15 tomatoes at once, instead of individually supporting them as they grow. You can do this in the 3x5 also but usually people do this when they have a long row of plants.

The support system is to have 2 strong poles at the end of each row and you would add twine or string at certain points along the height of the poles. As the plants get taller, weave their main stalk or other branches within this twine to give them support.