The Greater German solution is Germany which includes the lands of the Archduchy of Austria and Kingdom of Bohemia within a greater unified Germany. I’m not sure how it would go about but to my knowledge this is a compromise between the Habsburg and the Hohenzollern dynasties, so while Prussia would still lead Germany under this solution they would have a special relationship with Austria and Bohemia that is unique to the other German states, and the Habsburg’s in this case would only maintain their other crowns and lands outside of Germany in personal union, which might not require them to bring in Russia if the Hungarians still rebel because they would now have the entire weight of a unified Germany behind them, especially the southern catholic states.
The Middle European solution is if Austria herself unified Germany under the leadership of the Habsburg’s. In this case Austria would maintain all her other lands outside of Germany proper, not simply in a personal union. There would be a much larger German presence within the whole empire as a result of this and so the Habsburg’s could throw that weight around to intimidate would be rebels or crush outright rebellions when they occur, especially the Hungarians and other minority peoples in the greater realms of the Austrian lands.
The Habsburg’s were prime diplomats for most of their history, and unlike their neighbors, were never hegemonic and in fact promoted cultural and linguistic diversity among all their various peoples within their lands. They have great international prestige and respect and we’re never really a martial kingdom like Prussia. They could add a tempering nature to a unified Germany either ruled by Prussia or themselves.