Pattern sequences have about twice as many figures than a Scale Aerobatics sequence. Pattern is focused more on complexity derived from geometric design of the figures in the sequence, whereas Scale Aerobatics copies the IAC as much as possible, and where the sequences do differ, it often is done by adding a lot of snap rolls into the figures to keep the K-Factor (difficulty factor) up.

There is a well defined aerobatic box used for Pattern flying that you can actually see as a pilot and as a judge, and which is used to objectively aid in scoring figures (it's easy to see if the model has stayed within the box or not, hence associated downgrades are easy to apply if warranted). The box has three zones, which are defined by 60 degree lines to each side and a center line going out away from the pilot station. For the most part, maneuvers are judged for accuracy in a similar way between the two events, but in the case of Pattern, center zone figures are required to be centered and are judged accordingly.

In Scale Aerobatics, there is no scoring criteria for centering a center zone figure; you can place it somewhere in the center zone, or even out of the center zone if you like, and leave it up to the individual interpretation of the judges as to whether it was a good choice or not, as part of an overall impression they will develop of your flight. This goes for the the other figures as well, since they have an extra "figure" added to the score for airspace management that allows the judge to subjectively decide if you flew the entire sequence as they think it should have been done. Since there are also no box boundaries with physical markers in use during Scale Aerobatics competition, the location of the "end box" figures that are used as turnaround figures can also be placed to your liking, which may not be the same as the judges. Apparently it's not a popular concept in the Northeast at least, and not every contest director will elect to follow this rule, so you might want to check beforehand.

What's more expensive? That's mostly up to you, but going "all-out" in Scale Aerobatics takes a lot more money that Pattern. Both can cost you a bundle, however you can truly be competitive in Pattern for under $1000 in an airplane, even less in the Sportsman and Intermediate classes. That $1000 airplane can take you right into Masters class though. You could use either of your existing airplanes to start in either Scale Aerobatics or Pattern. In Scale Aerobatics, anything AMA legal goes in the Basic class. After that, you need an "IMAC Legal" model of some type. In Pattern, any fixed wing model is legal that is 11 lbs or less unfueled (except electrics which are weighed with all the batteries installed), has one engine or motor (glow, gas or electric) and is 2 meters or less in length, and in span. Many Pattern contests will waive the 11 lb rule for Sportsman class and allow a 15 to 16 lb model to be flown, to let newcomers get started without buying a purpose built Pattern airplane.

There are also noise limits defined for both Scale Aerobatics and Pattern, but they are generally not enforced except at large meets, such as regional or national level championships for example. Peer pressure will generally get you to quiet down your model in either case; it just isn't likely to be measured at most contests. IMAC has a subjective in-flight noise score, which you can look up in their rules (too much to describe here). Apparently it's not enforced much anymore either at local contests.

What should you pick for a new model or engine? Many, many good choices exist for either Pattern or IMAC, so it's probably a good idea to get out to a contest and see what's going on. Bring what you've got and try it, then you can get a lot of first hand input from the guys that are there flying other equipment. For Pattern contests in NSRCA District 1 you can see the remaining contest calendar at http://nsrca.us/d1-pattern-contests.html. You just missed a good Pattern Primer on Staten Island earlier in the month, but there is still another contest in September not that far from you in Jackson, NJ