Mainly it's how much wattage the soundboard is putting out to the speaker. Ideally, you would want the RMS wattage (which is MUCH less than peak wattage) that the soundboard puts out to equal the RMS wattage that the speaker is rated for, AT a specific Ohm rating for both. (RMS is less than Peak watts. Peak watts being how much the speaker can handle before going "poof!"). Using a car stereo amplifier and car stereo subwoofer as an example, let's say the speaker is a single cone speaker rated at 4 Ohms with an RMS rating of 300 watts and that it is the only speaker connected to the amp. In that situation, you need an amp that states that it can handle 4 Ohms and that states that it delivers around 300 RMS Watts at 4 Ohms.
With the little speakers we have in soundboards and with the soundboards we have available, we don't really get those options. We might be able to choose between 4 and 8 Ohms and 1 to 3 watts for the speaker but that's it. We can't choose the wattage and Ohms for the soundboard. Basically you just have to get whatever speakers the soundboard supports (Ohm and wattage wise) and that the soundboard creator specifies will work for his/her soundboard. I don't mean name brand though and some name brands are known to work better than others. I have used Regal speakers several times and think they are fine. I've never had one blow out on me. However others have and a lot of them prefer the Railmaster brand over Regal. As for which sounds better or louder, I don't know. I have four Railmasters sitting here that I have not used yet.