Thumb-breaks are generally faster than the other retention straps that use a snap. When the thumb hits the strap, there is very little pressure needed to have it pop loose. If you try one you will most likely decide that it is what you want. They are not that great for ease of one-handed re-holstering though.
The holsters you posted seem to be well-formed to the gun, and I would say that they do not need a retention strap. However, getting a holster with a thumb-break as shown would not likely add significantly to draw speed.
Not often mentioned by religious zealots: Numbers 31:17-18
Last edited by dahermit; December 29, 2012 at 12:14 PM.