Steve,
I fly in both types of contest, write in pattern, have covered an IMAC nationals, flown in two of them, competed in 12 pattern Nat's, judged AMA, FAI and IMAC, with a judging session at the Don Lowe Masters this year, so I might just have an opinion on your question. :-) (VBG)

I think that the differences are very few if you think in terms of aerobatic schedules. They seem greater when you look at the equipment but that is the result of two different design demands. Scale aerobatics, usually called IMAC have planes that have to look like scale aerobatics planes. Precision aerobatics, usually called pattern, have planes that can look any way they want but must not be bigger than 2 meters (78-5/8") or heavier than 5Kg (11 lb).

Entry level planes for both are now very much available at prices that are very reasonable.

The contests feel the same when you compete. The judging has the same ups and downs in both. The skill levels are very much the same. Interestingly enough, the top guys in both types of aerobatics are the same guys.

IMAC has a freestyle component that allows 3-D maneuvers to be flown and enjoyed. International Pattern has Artistic Aerobatics (FG1) but it has not caught on here yet and may never do so.

I advise anyone who wants to fly IMAC to fly it and the same for pattern. You develop skills flying either type that just don't or won't come to you when sport flying. Once you get into aerobatics you can experience crossover to your hearts content. Fly either or both.

The only bias that I really have is that I like the quieter planes.

Eric.