Krema, with all due respect, I suggest that you might be overthinking this... As most posters have said, specialized macro lenses are generally optically excellent, so no matter what you pick, it won't be a huge mistake, and if worse comes to worse, you resell one lens for at least close to what you paid for it and you try something else. I suggest that you get some practical experience under your belt to see what works for you. You're also apparently unsure of how depth-of-field issues vary depending on the type of shooting (birding vrs macro) and the settings used, so you might want to read up on that.
Personally, I think as long as you're buying a well-rated macro lens, the inherent optical quality of the particular model vrs another is not something you should be overly worried about. You can focus on other features. Longer focal length (90-200mm) is an advantage for bugs, but can make it harder to take pictures of larger flat objects (eg paintings) in tight spaces (eg artist studios). On another front, is a lens that only goes to 1:2 - like the Pentax-M 50mm F/4 macro, a very fine lens! - sufficient for your needs? (And if you don't know what "1:2" means, you need to read up on macro reproduction ratios.) Also, do you need/want a lens with a focus limiter (useful when using a macro for non-macro shots)? Or auto-focus even? What about WR? Pentax colors? Is weight a concern?
Some people pick up a macro lens, try to shoot macro, get awful results, and go on to blame the lens. But macro requires technique & patience. Even with a good lens, if you're shooting live critters handheld, it's rather difficult to get a sharp, well-focused shot with good depth-of-field at 1:1 (or above). But good sharp results
can be achieved with a great variety of setups, of which bona fide macro lenses are just a subset.
However for now, if I may, I think you should just pick the spot where you want to jump into the pool, and go! Buy a lens and go shoot some macros.