[…] First, what are the basic difference […] I primarily shoot in local, friendly matches. […] crafted in IDPA style. […] I much prefer wood to the plastic. […]
First, the basic difference is the frame back strap, or lack there of. Ignoring fitting individual slides to frames, both frames use the same slide. Also ignoring variation between individual pistols, 645s and 4506s have the same trigger pull. Both pistols use the same internal parts linking your finger to the hammer, except for their sear springs which seat against the different back straps. The 645 retained a traditional metal mainspring housing/back strap. A 4506 main spring's bottom seats in a blind hole in the back of its stock (or grip). In 3rd generation construction the rear of the grip frame was shortened and the mainspring housing eliminated reducing trigger reach. The shorter trigger reach provides better leverage during the DA pull. IMO, the basic difference comes down to choosing between a 4506 that's easier to shoot accurately DA or the better looking traditional 645 frame.
The 645s traditional appearance is enhanced by wood stocks but remains compromised by the its ugly trigger guard. The 4506's trigger guard changed to oval shortly after they were introduced.
One nifty 645 feature was lost in the 4506, its S&W revolver red ramp front sight. 4506s have modern 3 white dot sights. Depending on your eye sight, the lighting at your range and whether you local matches include low light stages, the difference in sights could make the biggest difference in your scores. Although if they haven't been relamped they'd have burnt out by now both fixed and adjustable sighted 4506s were some times factory equipped with tritium night sights. If you need to add them night sights are more readily available for 4506s. Also, unlike 645s, different heights of front sights are still available from S&W to sight in fixed sight 4506s.
Additionally the 645 had a minor flaw you should look out for when purchasing one. If lock tight wasn't used the manual safety lever retaining screws backed out. It's common to see 645s for sale that have lost one safety lever. S&W refuses to sell replacements. They want those 645s returned to have their safety barrel assemblies replaced with 4506 type assemblies at a cost of over $100.
So far I'm the only poster leading you toward a 4506. I think it's the most practical choice, but I own two 645s and only one 4506. In fact I recently purchased 645 checkered walnut from Nyteman 44 because like you I prefer the look and feel of wood over plastic. I hope this reply helps you make your choice. I don't have to decide. I'm not selling my 4506 or 645s in my lifetime.
