Why not buy several, test them out, and only keep the best? The difficult part is selling them off and not paying too much for them. If you keep an eye out, you can pick them up cheap, there is a lot of variance in prices on evilbay.
I went through a macro lens dilema recently, and nearly bought a Tamron f2.5 90mm (first version). It sounds like this lens also doubles as a nice portrait lens, and is more economical as there are loads available. In the end I opted for a 50mm M, which I am pleased with and a M 85mm f2. The fast aperture on a macro lens is only really useful for focusing, as you will probably need to shoot around f16. The difference between f2 and f4 at macro distances is negligible, the depth of field for both is very shallow.
Handheld macros lose resolution, even if you are shooting faster than 1/250. So photomacrography requires a tripod at least, and I think a macro rail would be desirable.
Have a look at this (old, but the arithmetic still applies) discussion of resolution and camera movement:
http://web.archive.org/web/20000612135202/http://members.aol.com/daveswager/resolut.htm