Which macro lens?

As above, which one would you guys recommend?

I already have a 100mm f/4 manual focus Pentax M Macro on the way, but it is only 1:2 magnification.

I am getting good deals on a D FA 50mm f/2.8 (1:1 magnification), and a Sigma 105mm f/2.8 (1:1). Both these new lenses are AF as well, and make good portrait lenses.

However, to throw a spanner into the works, there is a good Vivitar 2X Macro TC out on eBay for a low amount. Attached to my 50mm f/1.4 lens, it makes it into a 1:1 macro 100mm f/2.8 lens. Good reports for this out on the Interwebs.

Decisions,decisions! The D FA and Sigma lenses will run me ~ 300USD each.

This is for both flowers and insects.