You've asked a very important and insightful question about turkey chokes. Many times we refer to chokes by the exit diameter, for example, .555 or .570, when in reality the exit diameter is only one part of the internal geometry of a choke tube.
Each designer of a choke tube has his own idea of what works best and those ideas can differ substantially from maker to maker. A choke can also work very well with a particular shell and not so well with another shell, due to the differences in the designs of the shotshells. The easiest example to cite of that is the Flitecontrol Wad shells from Federal.
Those shells do not like chokes with ports because the ports will "eat" the petals and can cause the wad to fly irregularly and/or inconsistently. So, 2 chokes with the same exit diameter, one with ports and one without, may perform differently with the Flitecontrol Wad shells.
There are many other possible differences in the internal geometry of chokes. One may have a longer parallel section than another, for example.
Two chokes with like exit diameters may have different "internals" and may be ported or non-ported and may perform quite differently with the same shells.
Price does not necessarily dictate performance in turkey chokes. Many lesser priced turkey chokes will outperform more costly turkey chokes, with the proper shotshell. It's really all about the "combination of ingredients", the shotshell, the barrel (length, condition, etc.) and of course the choke tube.
You can read some information about my tests last year with various chokes and shotshells at http://allaboutshooting.com/article_info.php?articles_id=281
The good news is that there are several good loads and turkey chokes for your gun. It's just a matter of putting it all together.
I hope that helps some.