We use tatami omote from Mugen Dachi exclusively. I've tried the beach mats and, while definitely cheaper, they don't provide the same level of feedback.
The beach mats are I believe much softer. I tend to think of the whole rectangular piece that goes into homes as tatami, and just the outside as goza. I guess it depends on how thick you roll it, but how much would one of those bundles of tatami omote used for test cutting run you? I went to the mugen dachi page and read about the test cutting, which was informative, but the pricing pages went to someplace else. Looks expensive, and something that you wouldn't do at every class.
I wrote an article called Comparison Chopping (yeah, bad sense of humor!) for the Iaido Journal at EJMAS a number of years back (2003 or 2004?) comparing the different targets available at the time for tameshigiri.
Is it online?