You’re best served by avoiding 4 ohm drivers if at all possible.
8 ohm loudspeakers present a much friendlier load to an amplifier, and will provide you with a project you can live with over the long run.
In regard to 6.5" vs 8", it depends on your project and the loudspeaker cabinet you will use. If you intend to use something to augment the low-end, a 6.5" will do a lot better. Conversely, if you intend to use this loudspeaker on its own, an 8" deserves some thought, even if some will disagree with me on that.
Finally, to address your previous question about pairing a mid/woofer of 94 dB (per 2.83V, I presume?) sensitivity with a tweeter of 91 dB...in the real world, that rarely occurs. As for me, I've not faced that sort of situation in over 30 years of personal and corporate loudspeaker builds. In the specific case you brought up, because it was a 4 ohm driver with its 8 ohm sibling offering 91 dB, without digging further, I'll guess the former is drawing twice the power, and hence the difference. Comparing apples to apples, and that difference in loudness disappears. In fact, with a lot of amplifiers, the 8 ohm can actually do better. But for sake of argument, the first thing I'd want to look at is the actual SPL graphs, as manufacturers tend to slap a more than optimistic value on the actual spec, and the graph shows it as such. By this I mean, a driver spec'd at 90 dB/watt may actually play at 85 dB for 90% or more of the area under the SPL curve. But again for sake of argument, I would reduce / eliminate the resistors in the tweeter circuit to handle this scenario, and in fact have built loudspeakers without resistors on the tweeter leg. Anyone who has listened to what a resistor does to the sound understands how much damage even the best of them (and capacitors an order of magnitude or two even worse) impart, so I would consider this a blessing
