Pocket Nightmare is the original. Max Maven has been performing the effect since the 70's but has not released it commercially until recently. Since performing the effect for magicians over the years a few different versions have popped up. It's important to realize that Max's is the original.
Pocket Nightmare is half the price of Get Sharky.
Pocket Nightmare has been printed on a regular Bicycle Rider back deck. It has an air cushion finish and the cards feel the exact same as a normal deck of Bicycle 808. Get Sharky was printed on Pheonix backed decks. The cards are slippery and a bit thinner than regular cards.
Pocket Nightmare uses one force card, this means that the pocket is compleltey empty with the exception of one playing card. You do not end so clean with Get Sharky.