While Twitterrific seems to have more precise locating abilities, Twinkle uses the location-based tweeting in interesting ways. For instance, you can see tweets originating within a certain mileage range from your location. Twinkle allows you to easily view your direct messages.
Overall, Twinkle looks like it will give Twitterrific a run for its money. Best part of all? Twinkle (iTunes Link) for iPhone and iPod touch is absolutely free.
Check out our gallery for some screenshots.