For the Paragon, I am going with the second design. As well, had to share the fluff I just came up with for it, as I am particularly proud of it.
First check out this article stub for a little background
http://wh40k.lexicanum.com/wiki/Hundred_Planet_RebellionThe Paragon-class Battlecruiser was initially designed as a planetary assault ship for use within the Imperial Navy. Outfitted with the mighty bombardment cannons found only on the vessels of the Adeptus Astartes, the Paragon would have been able to bring entire worlds to heel within a matter of hours. Once in orbit, only the most fortified worlds, such as Cadia or Boros Prime, would be able to withstand a prolonged bombardment. The original examples of the Paragon were given initially to Imperial Governors bordering the Veiled Region in Segmentum Pacificus in an effort to increase stability in the region. Unfortunately, nearly all of the planetary governments in possession of these mighty vessels were swayed by a mad priest named Alldric the Subverter, and then used them in mass genocides or subjugation of rival worlds. This plunged the entire region into what became known as the Hundred Worlds Rebellion.
Imperial efforts in quelling the rebellions were spearheaded by the Dark Angels Chapter, along with nearly all of its successors. It is unknown why the forces of the First Legion descended en-masse upon the fallen sectors, but it is well documented that they were horrified to discover vessels of such might in the hands of Rebel Navy elements. Effort was made to capture these vessels, all of which were added to the Chapter Fleets.
After quelling the rebellion and knowing that the vessels had been instrumental in the fall of the region, the assembled Chapter Masters of the First Legion went to Terra, supported by representatives of no less than eleven other Space Marine chapters from outside the legion. The entire body of Astartes marched into the chamber of the High Lords and demanded that all remaining Paragon-class, both completed and under construction, be remanded to the hands of the Adeptus Astartes and never again be constructed, least of all for use within the Imperial Navy or Planetary Defense Fleets.
Face with such a show of force, the Council of Terra gave into their demands, the Fabricator General himself assuring Azrael and the other marines present that the plans for such fearsome vessels would be locked within the deepest vaults of Mars.
However, how secure these vaults truly are is in question, as at least one example, the
Steel Rain of the Iron Hands Chapter, was never accounted for in the original wave of construction. It is possible that the chapters closet to the machine cult have found a new weapon to wage war with.
