<font color='#000000'>Metal domes are more difficult to execute properly because they tend to resonate more severely and at a lower frequency than their soft-dome counterparts. In lesser designs, this can manifest as an audible peak or "ringing" in the top octave. It's a lot easier--and cheaper--to make a soft dome sound good, and that's why less expensive speakers tend to use soft domes.
While most cost-no-object speakers use metal domes, there are exceptions. Because the metal dome resonance can never be completely eliminated, some designers prefer to use soft domes instead. Huge generalization: metal domes sound more detailed but can sound harsh; soft domes sound smoother, softer, less detailed.
None of this should matter particularly to you, whether you're into high res formats or 8-track tapes ;-) Choosing a speaker based solely on the type of tweeter it employs is pointless. There are too many other variables. As far as frequency extension goes, different speaker manufacturers measure it in different ways, so it's sort of like comparing apples and oranges. Most high quality modern speakers have usable output to at least 20kHz on-axis. If treble reproduction particularly concerns you, the best thing is to listen carefully to the speakers you're considering, using program material you're familiar with.</font>