Theres already a number of Nelson vs. Hood threads, go check them out as well.
The lvl64 15" Mk III gun truly is a workhorse, having significant range advantage among other RN guns at BB3 level. But, the 15" ammo does not have that 'bite' that a BB player needs, compared to a blockshot 14" gun, or the high damage 16" guns. In that respect, I'd recommend getting the Nelson for its fine 16" guns.
Then again, I chose Hood because I was so used to the angles of the 15" guns and I wanted to have a different ship because everyone was automatically getting a Nelson, 'just because'. Beware though, Hood can be slow and has a big hitbox. If you have played KM BB before, then the 15" guns and this ship plays like a KM ship.