I'm currently running a 7 and 9 inch Lowrance HDS Gen 2 touch on my boat. I have been thinking of  changing to the Elite Ti2 because I can get a good deal on a 9 and 12 inch. Which is the better unit? This is my first experience with marine electronics. 

I don't have personal experience with those specific Lowrance units, but I have been using Lowrance electronics for 21 years now.  I currently use a Carbon and an Elite.

 

I would say the Elite Ti version is better at this point.  They are compatible with a Lowrance Live system should you ever want to add that.  If you are happy with the HDS Gen 2's and really have no interest in upgrading or adding on more accessories, then stick with the HDS ones you have now.  The Carbon I have is an HDS version, its just newer than the Gen 2.

I'm not sure if you can link the TIs together so I'm still running my older hds gen 2 units. I did however upgrade and purchase a garmin livescope  system.