The Z48/Z49 and Z58/Z59 do not have the same click mechansim. There is a difference in the feel of the click switches. The Z48/Z49 Clickies are the same as the SW02 for WeaponLights - this mechanism is built for battle and it is very popular.
Here is a photo of the shroud removed.
It isn't that easy to remove the shroud and I have failed in my attempts to replace those I have removed.
The Z58/Z59 appear to have been created after the "E Series" Z57 Clickie - the aim being to offer a more compact clickie for the standard body SureFires [such as the 6P and the M3 etc)
The "Z58/Z59" clickie TailCap is used in Black HA by the U2.
I don't have a Z59 - you may notice that the TailCap labelled in my photo above is Black HA when you compare it's finish to the Z49...
Anyway, Some models look better balanced with the Z48 compared to the Z58 - for example the M4 I think looks better with either the Z48, or in fact, the SW02. It certainly helps grip and activate it. The shroud also offers some impact protection if the light is dropped (although the M4 is likely to hit bezel first in the majority of cases).
The other day two Z58's arrived - one of which has the most amazing touch - significantly softer and almost silent click mechanism. Depending on your requirements, a positive, harder to activate click mechansim could be more desirable because it is less easy to accidently click giving you the ability to use the momentary on/off more easily.
An important aspect of SureFire's click switches is that they allow momentary on/off use of the push button pressure switch. I find this a very valuable feature. I am surrounded by click switch flashlights - I remember telling PK that I didn't like click switches. It turns out I just didn't have experience of well designed ones until SureFire introduced them to their lights.
Al