If the lakeview maps are the same version on the built in maps and the card, they will have the same contour lines for the same lakes. The card may have some extra features and add-ons - like satellite views or other info that is not on the built in maps, but no additional depth/contour lines. What extras the card has will depend on the version. Check the map options on Garmins site for details.
Your 2015 exchomap cannot be up-dated to the newest maps - it doesnt have enough internal memory to work with the new map format. You can up-date the card. The cost will depend on the version you have.
You may want to check on Garmins maping site to see if the lakes you fish have been up-dated since your card was released. You can look up individual lakes to see if they have new info compared to your existing card. If the lakes you fish have not been up-dated, there is no reason to up-date the card.
http://lakes.garmin.com/