Most people would probably say that there is no difference. However, my experience has shown that oval speakers are more prone to distortion at lower bass frequencies. As a rule of thumb I would go with round speakers. It also depends on the cone area and materials being used. You can certainly buy high end oval speakers that are less likely to distort given the higher quality materials that are used to combat distortion. Hope that makes sense. That said, why pay more for an oval speaker when a round speaker will do. My $.02.

When speakers distort it is often due to over driving them or having an incorrect enclosure for the given speaker(s). With round speakers you will have much more high end choices then the typical 6x9's, 5x7's or heaven forbid the 4x10's (but they do exist, just not as common). And with the higher end speakers you usually have a better product with specifications that can be used to properly build an enclosure. Watch out for B.S. ratings like 500 watts that some of the weak companies claim (usually lower end audio products) as that is something that will get you into trouble thinking it can run power if not a quality product. For bass speakers moving air AND control of the cone are a few of the important parts of the package. Without a stiff cone, quality surround and voice coil (as a package) you will have a low quality speaker even it round. Most 'quality' installers will have a sound room where you can crank a sample system. That is the best starting way to check out a system.

In most cases just like yours where you are trying to decide on a round 5.25" versus an oval 5" x 7"....here are a few things to think about. First, always try to sample a set at a local Best Buy or somewhere that you are not as pressured with salespeople. Use the same source and head unit and try to sample the same "series" of speakers; between the 5.25" and the 5" x 7". Normally, a company will put the same magnet (motor structure) on both speakers. This will definitely give them two different sounds, because of the weight difference of each speaker's moving parts. It is ALWAYS up to your ear to decide which one you like. The 5" x 7" has a heavier total moving mass and has the same "horsepower" that the 5.25" has. This will change the overall sound....but you may very well like it over that of it's little brother, the 5.25". More bottom end or "bass" should be realized with the 5" x 7" while its midrange will be more mellow than the 5.25". Try 'em both and let your ears decide for U!

In general, I prefer round speakers. It puts a more even pressure and stress on the cone evenly, it just seems more natural. And now with square, pentagon, triangle and every other shaped speaker available, I still prefer round. That is a complete personal preference, the way you like them to sound is what matters most, it is your vehicle.