Hi Leon
I have owned and used the Canon EF 100mm F2.8, Canon EF 100mm F2.8 L IS USM (both 1:1) and now the more recent Canon RF 100mm F2.8L IS USM (1.4:1) macro lenses.
Even pixel peeping, with good technique and lighting, I could not tell the difference between the non-L and the L versions of the EF Mount lenses.
The Canon EF 100mm F2.8 L IS USM certainly felt more solid.
I tend to stick with Canon because I once purchased a non-Canon brand lens and the focus ring rotated in the opposite direction to all my other Canon lenses and I never got used to this.
In terms of flash, this is my current set up. The Speedlite is diffused by a Lumiquest diffuser, then by the translucent plastic “wing” fixed with Velcro to the lens hood which further diffuses the flash. This gives me nice, wrap-around shadowless light and minimises the risk of hot spots on shiny insects.
If you are going to doing dental work, then a simple ring flash might be better.
Cheers
Dennis