Are you planning to sue it to run offshore, assuming due to your local your looking to maybe run the mudhole and further points? An in deck tank is best, but as mentioned will cost more, and will have the biggest effect on ride due to location, it also is more maintenace since you need to now run that fuel and keep it fresh, and not let it sit to long, especially empty unless you want water to build inside the tank. I'm sure Grady could send you their aux tank and place it right in, or you could have a custom one made for probably around the same price and maybe squeeze in a few more gal. An on deck tank will cost a little less and quick disconnects make it easy to use, problem is storing them after you run them dry. Always run a bladder tank first, for starters you do this to obtain your best performance, the location and weight of a bladder tank will hurt you, but some aux tanks do as well since they are so far fwd on smaller boats. You could roll and store the bladder tank forward on the walkaround but I could highly recommend getting a cover for it to keep salt spray off it during the ride in. I know a buddy who runs a 29 welcraft and he has a 100gal bladder tank, quick disconnects make it super easy to attach, easy to switch on when needed and low maintenace since the tank is stored at home off the boat away from harsh conditions when not used, also keeps the boat lighter for typical use and was much cheaper initial cost. Down side to a bladder tank is life span is typically not as long as a permanent tank under the deck, but that is assuming the deck is sealed well and corrosion is not eating away at that tank as well.
If you plan to use the boat a lot and see need for the extra fuel and will see it as something you will use for maybe 8-10 years, maybe the in deck tank is the way to go. If your looking at maybe 2-3 trips a season and don't know how long you will hold onto the boat before you might upgrade in size, the bladder would surely be cheaper and suit your needs.
Also consider where you will store the tank after it is used and if this hinders how you plan to use it. You might be able to have a nice storage compartment built where the aux tank would do in deck and have a drop in bid and put it there, and then put in buckets and cleaning supplies when your not running further distances and carrying the extra tank.