Lavalier mics are handy when you need something discrete, particularly for stage and TV work.
On the high end (over $400) you've got mics like the Sennheiser MKE series and the DPA 4060/4061. You can save some money by going for the slightly less exacting Sennheiser ME 102 and similar.
A good lav in the mid-range is the Audio-Technica AT899, at $275. Remember though that if you want to record the interviewer as well you will need two of them!
For impromptu situations, in-the-street interviews and the like a battery-powered single-point stereo mic like the AT822 ($250) is a common choice.
Better yet, just do like the BBC do, get a Beyer M58 ($180) and forget it! This mic has a long handle for getting into people's faces and a hot output to handle weak inputs.
I'm not going to specifically address the mics from the two companies suggested by the poster because at the lower end I do not imagine them good value, and at higher prices why not just get name-brand professional kit?
One thing you may not know is that many lower priced mics use a Panasonic WM-61A omni cap. You can buy these raw in prices at around $1. By the time companies get through with them they cost much more, but I guess that's "value added"! The best thing to do is purchase mics built from these caps from individuals on eBay. I can recommend seller micro_sound who offers a binaural pair at the fair price of $23.
A cheap set of mics like this is good as a "reference standard" in the low-end, and as a pair you can throw around and not care if they get wrecked, lost, or stolen. They are surprisingly usable for tasks like voice recording.
Disclaimer: I have not used many of the more expensive mics mentioned above, but instead have based this article on best practices recommended by others. Also, I don't know how any of these work with the MicroTrack. It's always best to test mics with the specific gear (power, recorder) you are using.
Further reading: A Transom article addressed a similar issue.