The lounge on Fantasy in enclosed in the middle of the 12th floor. It is a bit nicer because of the wood paneling, but it severely lacks windows compared to the lounge on Magic. The Concierge sundeck on Fantasy is upstairs from the lounge, so it has a lot more room than the current outside area on Magic (which should eventually be expanded during the next dry-dock), but, again, it is enclosed in the middle of the 13th deck without a view of the sea. We rarely see people hang out there outside of the character meets or maybe during happy hour.
When on Wonder and Magic, we spend a lot of time hanging out in the lounge or the shaded seating outside, but we find that we always go back to our suite on Fantasy with our snacks and drinks because we like to look at the sea or the port.
Other than that, the amenities are the same, including the food station and the coffee machine. They sometimes hold private concerts in the Fantasy lounge, while they use reserved public lounges on Magic and therefore only hold them during the Concierge reception.