Shelley's conception of nature, as presented in Mont Blanc, hinges on an essential perceived duality- what is sublime against what is "ghastly, scarred, and riven." That the Ravine of Arve is referred to as "that, or thou" is significant- "that," third-person nature, or "it" nature, can be taken to signify the ghastly, scarred, riven aspect of this "clear universe of things"; "thou," second-person nature, or "you-nature," can be taken to signify the natural majestic or sublime, companionable and personal against "it." Shelley has, at his disposal, models and/or conceptions to gauge what best represents the Power or "secret strength of things" which is seen to under-gird both nature and human thought- the mind's musings on itself (self-reflexive musings), or the equally self-reflexive pursuit of poetic/creative "ghosts," or a language/linguistic universe. That Shelley opts for visible, material Nature, in its duality, as the most workable model or synecdoche of this Power indicates that Shelley's conclusions seem to follow an imperative drive towards the crowning of empiricism or materialism over imagination, in a manner that Kant might approve of. It is the streak of a scientific ethos in an aesthetic context, and purifies Shelley's conclusions: re-affirmations of duality.
Keats, in comparison, likes things companionable all the way through. He attempts to impose "thou" status on everything, and to live in, and write from, a resolutely personal universe. Not just personal; a personal universe tinged by imagination into an ultra-personal, or hyper-personal universe. The first line of Grecian Urn, "Thou still unravish'd bride of quietness..." gives (to some extent) the entire anti-empirical game away. Oddly enough, Keats' connection to "that," to a third-person situation, context, or universe, is manifested by the mysteries of his prosody- where it comes from, its power and secret strength, how it manifests. It is not, it must be noted, particularly accounted for by Keats himself; he is, in Romantic terms (apropos here), the conduit or channel for it, and absolved by this position from the rigors of having to account for its empirical manifestation (or, as they said in Regency England, "numbers").