Get oval speakers if that's what came from the factory, unless you want components. Unless you have other speakers up front you want the 4x6 to get more bass on the same amount of power. Otherwise you have to get 4" components because nobody makes a 4x6 mid woofer.
There are reasons an oval speaker might sound worse than an equivalent round speaker, but I can't say I've ever seen anyone do an in-depth comparison. They sound fine to my ears.