Have had both types.
1. Welded is probably stronger. Plus, you don't have dissimilar metals together [aluminum and steel bolts]
2. I tried galv. wheels. Too much floatation with the tires and the surface of the aluminum "I" beams.
3. I added a PVC pipe I think it was 3 or 4 inch. Filled it with water till it stopped the floating. But, you're loosing your weight advantage. I ended up taking it off and just putting up with the floating trailer.