The down and dirty answer is this: Typically, true three way boxes have a narrower pattern that is usually 60 X 40. This is par for the course in the MI world. If you spend more money you can get narrower patterns. Now for quasi three way boxes such as the DSR215, that is a different beast. It is not actually a three way box. It utilizes two 15" speakers one of which is crossed over to reduce low end interference. Hence the "quasi three way " description. The quasi three way box utilizes a standard horn for the highs and the 2 X 15" speakers simply fill out the rest of the frequency response.
In a true three way box there are three separate drivers each of a different size and all have independent crossover points. Since most three way boxes have a horn loaded mid driver, they match the highs to it so that multiple boxes can be splayed together. The narrower dispersion allows the boxes to work better together than a 90 x 50 horn. In essence two 60 X 40 boxes can achieve 120 deg. of horizontal coverage. Because the horns are narrow the speakers are in a closer plane to one another which means that box interactions are reduced in comparison to a box with a larger horn.
From an applications standpoint you have to know what your desired coverage area is. A 90 deg. box is best suited for simple setups where the coverage is most desired to be in front of the stage or in an application where you need 180 deg. of coverage utilizing a pair splayed together. A 60 x 40 box is best suited when you need coverage greater than 100 deg. up to 180 deg., or in reverberate spaces where more coverage will saturate the room. Keep in mind that three 60 x 40 boxes will achieve 180 deg. of horizontal coverage, but with increased interference over that of two 90 x 50 boxes. Ideally if you need 100 deg. of coverage you would utilize a speaker with a 100 deg. horn. At the MI and semi pro level there is not many coverage options other than 60 x 40 and 90 x 50. There is only a couple in that market range that will do 100 deg. or beyond and they are again limited to application.
Another thing to note is that a three way box will typically outperform a conventional two way in both terms of output and sound quality while also increasing control of more mid range frequencies vs that of a two way box. It should be noted that narrower horns do not throw sound further per se, but more that the controlled concentration of the sound they emit holds together better over distance. This is what makes narrower horns better for indoor reverberate spaces. The energy is more focused increasing intelligibility and reducing excitation / saturation of the room.
Since the mid horn is designed to have some control over the frequencies in it, there is more frequencies that are then controlled between the two horns. Frequencies beyond the horns cutoff point and crossover frequency are not under directional control. A typical 90 x 50 horn for the highs has a crossover point somewhere between 1khz and 3khz ( typically closer to 1.5 - 2.5khz ). Anything lower than the crossover point doesn't have any real directional control other than the beaming of the low driver producing it. So with a three way system where the mid horn may be crossed over as low as 250hz ( typically closer to 500hz ) and up to as high as 2khz, you can have increased control over a much larger number of frequencies.
You must first decide what your application will be and how much coverage you must truly have? If you need wide coverage a lot and do a bit of indoor stuff, the ETX-35P would be a great option if you could afford 4 of them. If you only do band type stuff in clubs or where your only really interested in the dance area in front of the stage, then the ETX-15P would suffice quite well if you can only afford two of them. Since you have a fair amount of subs, I would be more interested in the ETX- 12P over the 15" model if you just need dance area sound. The smaller the speaker, the better it sounds typically. The difference of 1-2db between the 12" and 15" model is nothing to worry about. Food for thought anyway.....
