I have an HB 596c HD DI and have found it to provide very useful information for the fishing that I have done so far. I had done LOTS of research on FF's before buying and found that this unit provided the features that I THOUGHT would be most important to me. The person that I purchased the unit thru tried to advise me to consider stepping up to, at the very least, a combo GPS unit and STRONGLY believed that I would be better served by an SI unit on my home lake [The Chick in Tn]. I am wondering now whether I should have taken his advise on one or both. My question , which do you believe is a must have or are GPS/SI equally important?