That is really up to you. Many people like the experience and the anticipation of hatching a shark from an egg. However you may find it very difficult to get your newborn shark to eat. It may possibly never accept what you feed it and it will eventually starve to death. It is a good idea to usually buy one that is older and eating regularly. It's probably impossible to see one eat at a LFS, but you can question the workers that care for the animal and inquire about its eating habbits and how often they feed it. Also look at its musclature and make sure it's not atrophying. If it looks very skinny and sickly, it probably hasn't been eating.
For the first time buyer I recommend a live shark. Your second time around you could try the egg.
Jim