Eksnipan,
Welcome to the forum. Since you only have a 798c, why not get another compact SI transdcuer vs the HDSI. You will get better results with dual matching transducers.
I cannot think of any benefit in going with the other HDSI transducers unless you are planning to upgrade to a 800 series or higher unit. If so, I would just go ahead and sell the compact SI transducer and get two matching HDSI.
Good Luck,
RGecy