No doubt, i would have prefered a set of MB Quart tweets, at the time of purchase, I couldnt bring myself to drop the coin, so i spent 1/4 and wound up with something that worked well. The EQ doesnt change the characteristics of the driver, true, I just foud that cheap little things sweet spots and let them sing. Also, I had them fiberglassed in the Mach door pods for a good stage. Alternatively, I was tempted to try a kickpanal placement, but ended up not keeping the car long enough. If you have a good tweeter budget, Get the best you can afford (especailly since these little things are responsible for so much) but on a PTimers budget, lesser with a EQ tweak is a close second.
As far as speaker advice, for the mustang there is a miriad of aftermarket drivers. Its personal taste that will ultimately draw you to what YOU like. Speakers are like Wheels, what one likes, the other may not. Just be aware of RMS power and decibel effeciency (SPL). Aside from that, choosing a speaker with rubber surround material is a good move for automotive sound (less or NO rot as in foam surround speakers).
If you can afford a tweeter/ 4" / 5 1/4" OR 6 1/2" , this triple combo yeilds the most believable sound stage, and produces a well rounded sound. I would also advise testing mids/highs with music that is primarily female voice / double bass (upright) and well recorded live music. Sarah McLachlan / Eagles unplugged version of Hotel California and possibly some R&B.
The female voice, for me, is what convinces me that a speaker is performing up to snuff accross a broad range. The rest is preference, but it is good test material I'll tell ya that. Anyway, hope this helps. As far as brand go, big dollar mids usually perform best. MB Quart / Diamond / Focal / and even this years Infinity isnt to shabby. Good luck
"Jesus has my heart, but Whiskey has my liver" --Jason Boland