I think both albums are superb, but I have to go for Machine Head.
Machine Head is my favourite Deep Purple album and is probably in my top 10 rock albums, whereas II is my third favourite Led Zeppelin album after IV and Physical Graffiti and would 'merely' be in my top 20. Also, Machine Head has one of the best side twos of any album - Smoke On The Water, Lazy, Space Truckin' form a superb trio of tracks to close the album.