I am familiar with all Deep Purple albums with the RB line up but with SM I only have two albums. Maybe its because I have not heard all the albums involving SM. The album infinite has four good songs on it and one song 'The Surprising' is half good, The Singing part is excellent but the instrumental part lets the song down which is such a frustration. I felt there was a lot of 'rock' rhetoric going on in most of the songs on that album and if the song itself does not have any redeeming melody,riff or anything then all that heavy rock rhetoric falls on its face.
As to favourite Deep Purple album, I don't have specific one because there are quite a few albums with very good songs on it. I think the album which which reflects the essence and mojo of Deep Purple is most defnitely Made in Japan.