Well, thank you for making me google "methane content flatus carnivore vs herbivore" first thing in the morning.
Couple of interesting points to consider before calculating gross pounds of dino flesh vs. steak over time:
1. Carnivore/Herbivore mix and estimated methane content of each
2. Diet (leafy green vs. grain)
3. Ruminant vs. Single Stomach
4. Domesticated vs. Wild (differences not accounted for by diet)
5. Other biological differences in digestion between avian/reptile and mammal
6. What microbes lived with the dinosaurs?
Any answer you come up with is going to rely heavily on a large number of assumptions. Not that it's a bad thing or pointless, but it'd be interesting to compare estimated results to what we actually know from empirical evidence if we have any.
Here's a couple of interesting tidbits (http://www.springerlink.com/content/n5171l64625lq6l1/
The predominant methanogen in all except the chicken and turkey is species of Methanobrevibacterium. The chicken and turkey harbour species of Methanogenium.
Methane production by monogastric animals is lower than methane production by ruminants. However, methane production by large herbivorous monogastric animals such as horses, mules and asses is substantial (up to 80 l per animal per day). Methane production by rodents and avians is low. In general, methane production by wild animals is lower than methane production by domestic animals. It is concluded that the contribution of monogastric animals to the global methane emission is negligible, as it only represent about 5% of the total methane emission by domestic and wild animals of 80 Tg per year.