It can be an overwhelming situation: You're stuck in an audio hardware store, taking a look at all the various speakers on display. Sometimes if you're lucky and ask nicely, you might even get to hear the speakers before dropping hundreds (if not thousands) of dollars on your perfect speakers. However, it's unfeasible to listen to every set of speakers to compare them. A sensible approach is to form a shortlist of potential candidates. Once you've narrowed it down to two or three speakers that lie within your price range, you can then think about tracking them down to listen to and, hopefully, purchase.
It also must be said that there are some key differences between monitoring speakers and speakers that are intended for mobile use. Additionally, there is no sense in buying stadium-sized speakers if you're doing relatively low-key parties that don't require acres of sound coverage; moreover, price aside, these speakers tend to have much more heft than is ideal. Whereas studio monitors are specifically designed with balanced audio in mind, standard stereo, hi-fi and car speakers are specifically made to give a richer, more specific sound, which can include bass boosting or the enhancement of highs or lows depending on the intended purpose. Conversely, studio monitors are supposed to give an accurate or 'true' replication of the audio it is interpreting.
In a mobile DJ setup, monitoring speakers — while typically giving an accurate interpretation of the sound — aren't usually the best choice for DJing sets (although not bad, of course). Also, it is also worth considering investing in a separate subwoofer to really make the dance floor shake.
Bear in mind, however, that certain buildings — especially heritage sites and multipurpose venues — will frown upon the use of subwoofers or heavy dancing or stomping. If you're using a new venue for your gig, it is best to check with the manager regarding the use of subwoofers. There is no sense in upsetting your venue owner over something that isn't an essential part of your gig. Keep in mind, though, that if you're, say, a dubstep, hip-hop, or drum & bass specialist, it is best to check beforehand whether the venue will be okay with your use of a subwoofer.
Finally, many venues will have their own speaker setup. If you are happy to go with their setup, ensure in advance that your cabling system is compatible with their speakers, otherwise it might be best to use your own DJ equipment and setup.