Both compliment each other - especially as redundant systems. We run both in our Ultra4 car and both have worked flawlessly. We have used LeadNav in our chase rigs because most of us have iPhones, but we also haven't done long chase races like the Baja 500/1000 with it (Hammers is our biggest race so far, and we've always self recovered).
If I were to pick one, I would go with LeadNav as an iPad has more uses outside of navigation for me than a Lowrance. I'll likely outfit our chase trucks for a larger iPad for this reason. If money were no object I'd run one of each.