What has better sound quality? Round vs Oval
Round!
Oval is usually only used by the OEM manufacturer's, and only reason there are aftermarket ovals, is for people who are too lazy to switch to rounds. The reason OEM's use ovals is because it is a trade off in sound quality versus output. Output is essentially Power X cone area, so if you can squeeze in a little more cone area by making an oval shape, you can get a little more potential output for the same amount of power.
What it comes down to is sonic physics, an oval speaker cone has a surround that is not symmetrical, this causes uneven travel as the cone moves in/out and uneven sound wave generation.
I have 6.75" mids in my truck and they fit with zero metal cutting due to the shape of the basket. I only had to drill 4 new holes for the screws. You should be able to get a 6.5" speaker in the same factor hole without any trouble, or even some 6.75" speakers, at the very worst you will have to widen the factor opening a touch, just in the center of the opening.
For me personally, if you are only going to stick to head unit power, especially the stock head unit, I would limit my budget to around $1-150 per speaker set if not less. The stock head units put out somewhere from 7-10w RMS (I believe) and an aftermarket unit will average 17-20w RMS (they will all advertise 50x4 peak, but RMS is all that matters.) Due to these power levels I think you will easily see diminishing returns in speakers on the higher end of things. If anything take whatever savings you get on the cheaper speakers to drop in a decent 50 or 75 by 4 amplifier off the factory HU.
Also due to the lower power levels the key spec on the speakers you should be looking for will be Sensitivity. The higher the number the better, it is basically a measurement of how loud the speaker can get per wattage.
All things considered being stock HU power, I wouldn't be too terrible concerned with the sound quality difference between round/oval, and you might benefit better from an oval such as those Infinties you linked. Also Infinity usually has a higher sensitivity.