I've competed seriously at both and agree with most of the posts above.

Pattern has a much smaller "box" to fly in. You only fly 1 round at a time, and as mentioned above, landing and take offs are a part of the sequence, regardless of wind conditions. Pattern is much, much more harsh on centering and in-box placement of the manuevers, as well as smooth and gracefull. However, pattern is only downgraded by the judges for 1 pt per 15 degree off. The planes are smaller, as a rule quieter, and as a rule cheaper(although this is starting to change)

IMAC has a much larger "box" with a tremendously deeper backside to it. I personally know of clubs that have been closed down because of this. Not a dig here, just a fact. I'm not gripping either because I like the huge rolling circle also, just can't be done at every field.
By the same token, jets can't fly at every field for different reasons alsol. In IMAC, you fly the sequence twice, yeilding 2 different round scores without landing. I have seen pilots let someone else land their plane after flying. The downgrade in IMAC is stiffer, 1 pt per 10 degrees off. IMAC has many more snaps/spins. IMAC also has Unknowns on Sun morning which can be a thrill.

Those are the differences, but look at my signiture which has been this way for 4-5 years.

The similarities are competition. A HUGE misperception is you can fly almost anything in either for Basic. you don't need an IMAC plane or a Pattern plane to compete in the entry level classes. Both groups have a blast. I beg to differ with the above on Pattern pilots. The pilots that fly Pattern (AMA) have as much fun, but fly more flights per day. On saturday, the contest is set to fly 4 seperate flights, whereas in IMAC they will fly 2 seperate flights, with 2 sequences each. Both have 4 individual scores.

I believe most pilots wanting to compete will typically gravitate to what is available, or what they see at their specific club. In some areas, pattern is thriving and IMAC suffering, and in others vica versa. There is lots of grumbling from either side of the fence. Such is life.

Try either, have fun, and you will probably fly better on a daily basis if you do.

ed