It's more of a budget consideration or personal preference than anything.
You can get some fantastic bass from 6.5 seperates. A pretty cool setup is seperates (not the same as plate speakers) in the front, with a 6.5 driver mounted in the stock 6x8 door location using a 6x8 to 6.5 adapter, and mouting the tweeters in the sail panels(inside of the side view mirror), then throwing a 2-way 6x8 or a plate speaker in the back for rear fill.
Probably won't notice any distortion unless you listen to really vocals-heavy music (I'm talking opera and solo stuff, not pop/rock/rap), and even then, it's more about the ability of the speakers to reproduce sound rather than their shape.
Do what works for you. I say 6x8's all around if you want a drop-in install and minimal expense for maximum value, and go with seperates if you just gotta have the best mids and highs you can get.
Something else to keep in mind is that most 2-ways and 6x8's will work fine with the factory stereo or powered off a head unit, where seperates require their own dedicated amp due to their higher power requirements. You can get an amp for 6x8s that have the power handling, for drop-in speakers with ear piercing highs.
