Prep Course: It is a good idea if you are starting out and want to be on the straight and narrow. It also serves to discipline your routine since you end up going to the class (thanks to the $$ you paid).
However not so great idea if you have already covered the basics and the particular course caters to those who start from the very basic.
Study Alone:It is a good idea if you want flexibility and are the type of person who like to study by yourself. Ofcourse you should take help of forums such as GMATClub

Not so good idea if you tend to be low on motivation, will-power - a few days of slack can easily snowball.
Study Group:It is a good idea if you are sharing experiences, getting better by explaining topics to each other, exchanging study material etc.
Not so good idea if you are dependent on others for your plan, or it takes up too much of your time during weekends.
Its upto you to take a pick

Arun