Yes, the 798ci si does down imaging as well as side imaging since early 2010 software updates.
The dedicated down imaging unit may have a little higher di resolution because it has a norrower beam, but a MUCH smaller search area.
I doubt that you will find anyone that has a si unit who would go back to not having it.
1 vote for the 798ci si.
Go with the 2011 798ci hd si if you can....it has the ethernet port and will be set up better for the future.