I think it would be surprising for them to announce one model and wait a few months to introduce a second. I think what's more likely is that if one is 'delayed', they'll just announce a later release date. I'm thinking similar to what they did with the iPad 3G. Was that the model I'm thinking of? It wasn't available on the same day as the WiFi-only model?
The iPhone 5s overshadowed the iPhone 5c because it was entirely new. The iPhone 5c was nothing more than an unapologetically plastic repackaging of the iPhone 5. That's the reason for the increased interest for the 5s over the 5c.
If there are two iPhone 6 models in different sizes, you won't see one completely dominate. I'd venture to guess most will opt for the 4.7-inch. I think the 5.5-inch, if it happens, will be like the gold iPhone 5s. Available, but limited in supply.
I'd rather order online from Apple too, but I have this $50 gift card from Best Buy. Last time I dealt with Best Buy (when I bought my 5S), they screwed up and didn't activate my visual voicemail, so I had to call AT&T. They got it straightened out, but still... Oh well. I guess it's worth $50. heh