I'll chime in here, even though I have not grown either of these grasses. I do have direct experience with several other Miscanthus varieties, as well as other similar warm season grasses, so I think I can help.

Pick the one you like the look of. Neither of them will be more prone to flopping IF you give them a suitable environment. That means good drainage, not-too-fertile soil, and plenty of sun (6-8 hours a day).

They do have somewhat different habits, and each has a different look. Compare them on this chart:

http://www.bluestem.ca/grass-comparison-chart.htm

By the way...there are some newer varieties, like 'Gold Bar,' which offer the horizontal striping, with different habits and growth rates.

I'd agree with donn that flopping is not a big problem if grown in the right conditions. However, I would say that Miscanthus Zebrinus (zebra grass) will be more at risk to floppy-ness ?:) than Miscanthus Strictus (porcupine grass). Zebrinus grows more arching to begin with, while Strictus has a very upright habit. I prefer Strictus myself...it does hold form better in my opinion.

Here is a link that might be useful:

Thanks for the information. I also read that porcupine grass is extremely slow growing, i.e. eight years to reach maximum height. Does this sound accurate to you?

Thanks again!!

No, I don't find Miscanthus 'Strictus' to be a slow grower. 'Gold Bar' (which is basically a compact Strictus), however, is extremely slow. That's ok, it suits it's usefulness as a compact gold striped miscanthus.