I have the opportunity to get both of these units for VERY cheap. Both are used but it great shape. Just curious s to which one is better, and if they could use the same transducer. It'll be going on a kayak, and idk how to mount them quite yet.

Both units will do the same thing... It all depends on which screen size you want.  They both use the same transducer and Lowrance offers many different options to use them in a kayak such as the scupper hole mounting system and a carry pack which will include a mobile battery and plug in charging system so you can turn the unit into a mobile unit.