My biggest knock against Carbon is its 15-minute refresh interval. As of now, the app has this rate set in stone, which could push many power users elsewhere. By comparison, Falcon Pro can be set to refresh as frequently as every 2 minutes. Another issue I have is Carbon's lack of an internal browser and in-app previews. With more than half of my tweets containing links, I would love to be able to get previews of pages without having to exit the app. Also, tablet support, landscape mode, and a home screen widget would be nice.
While Carbon may not be perfect, it is still one of, if not the best-looking Android Twitter client I've seen to date. With its minimal design, it has a lot more open space than other clients, and with its nifty animated details it just feels more polished. Carbon even supports multiple Twitter accounts, which Falcon Pro currently does not.
In the end, I actually prefer the look and feel of Carbon to that of Falcon Pro. I also love that it supports multiple accounts and it auto-completes usernames. However, I can see the set-in-stone 15-minute refresh rate being a deal breaker for many.