Hi,
I've been to both Demera and Ras Dashen, but only once each.
Qualifier: I've recently moved to Chicago from Washington, D.C. D.C. has a large Ethiopian population (second only to Ethiopia) and, accordingly, many many Ethiopian restaurants. I lived close to the main Ethiopian restaurant neighborhood and ate dozens of meals at these establishments. I ate Ethiopian like some Chicagoans eat pizza and hot dogs - - that is to say, I ate it quite often, sometimes more than once in a week.
That said, and knowing that I've only been for Ethiopian twice in Chicago, I feel compelled to say that Ethiopian is not one of Chicago's strengths (the reason why I've only eaten it twice here). Demera bordered on terrible - we couldn't get water or additional injera (the restaurant was nearly empty); and the food was greasy and the portions were teeny. Basically, the entire experience was bad and I have no intention of returning. It sounds like germuska had a very different experience, which is good to hear, because ours was so very negative.
I guess the bottom line is that I would go with Ras Dashen over Demera, without a doubt. I thought Ras Dashen was decent -- it satisfied a strong Ethiopian food craving. My favorite dish, lamb awaze tibs, was rather disappointing, but I really enjoyed the veggie dishes, which look to be your focus. BYOB at Ras Dashen is also very nice. I look forward to trying Ethiopian Diamond and Lalibela.
In any event, I hope you'll post after your trip. I'm interested in knowing which restaurant you go with and how you enjoy it. (And, if you ever plan a trip to D.C., I'd be happy to suggest some really special Ethiopian spots)