Both cartridges are very accurate at 100 yards. And in the power department, both will do a serious number on deer or even black bear size game. That is providing that you do your job. The .357 Magnum is the more elegant of the two rifles(broad power range and ease to shoot). The 44 Magnum is more of a sledge hammer. If you are in need of a woods gun to stop dangerous game or hunting larger game, like elk, the 44 will be a better choice.
To answer your second question about hot loads. The answer is yes and no. Yes, you can use a load that is hotter than what is typically available commercially. Modern Marlin rifles can safely take pressures to 40000 PSI (a bit more on the .357), most commercial ammunition is 3500 to even 7000 PSI less. No, you can't make loads that exceed 40000 PSI (about 42-43000 for .357 Magnum), it will lock the action up.
Robert