we only eat watermelon when it is available and that is not very often. I've never taste tested one against the other. It might be a fun test to try this summer.
I've never read that one is more nutritious than the other, but it's possible.
I think the regular seeded watermelons have better flavor and texture generally as compared to their seedless competition. But I think the seedless ones have been improving, especially the last two years.
The trick is to get the flesh to ripen before the seeds have formed up and they seem to have got the plant behavior down now. Because no melon is truly seedless.
As to health, you'll have to be specific about what attributes you think are healthy. I think they're interchangeable in all the general attributes.
The presence of black seeds in watermelon does not significantly affect its overall nutritional value or healthiness. Both black-seeded and seedless watermelons offer similar nutritional benefits and are healthy choices. The key nutrients and health benefits of watermelon. The main difference is the convenience of eating seedless watermelon, as you don't have to spit out or discard the seeds. Some people prefer the texture and taste of one over the other, but from a nutritional perspective, both types are healthy choices as part of a balanced diet.
I asked this question of both seeded and seedless watermelons and, by a large margin, the seeded watermelons said they felt extremely healthy, while their seedless counterparts said they felt like they'd had an involuntary vascectomy and they were sick about it.