I originally wanted to write a detailed and wordy comparison between the two with lots of screenshots, but I didn’t have time today and I probably won’t for a while, so here are my quick opinions:

Features:
Tweetbot supports more image/video/URL shortening services than Echofon does and has nice Favstar integration. It supposedly has better list support, but I don’t use lists so I can’t comment on that.

UI:
Echofon is very simple and clean, and uses almost all native UI elements. A lot of people like that. To me, parts of it (profile views being the best example) seem hastily thrown together, and other parts have a lot of wasted space. It also takes a really long time to get thing done in some cases because it’s a huge cascade of menus.
Tweetbot on the other hand, uses a custom UI. It’s more complicated than Echofon, but once you learn how to use it navigation is a lot faster. I personally like Tweetbot’s UI better, but a lot of people don’t.

Syncing:
This is where Echofon really shines, especially if you use it on your Mac a lot. Not only does Echofon have timeline syncing, but it will also disable notifications on your iOS device if you’re using it on OS X, which is really great. Their Mac app is also much better designed than their mobile counterpart, and I don’t have any gripes with it as far as the UI goes.
There is one problem I have with Echofon’s syncing: it syncs whatever the most recent tweet loaded was, not what was most recently read. This may not be a big deal for some people, but it was quite annoying to me.
Tweetbot uses Tweet Marker for its syncing. This is a good thing. Because it’s open and free for anyone to use, it can be built into any client for any platform. The only problem with this is that there’s a depressingly low number of clients that actually use this feature. On OS X you really only have Twitteriffic and a Chrome extension which adds it to the web view, and on Windows/Linux I haven’t been able to find anything but the Chrome extension. Twitteriffic is a good Mac client, but one thing that really sucks about it is that it doesn’t support the live stream tweet API, so you have to keep refreshing the tweets or wait for its auto refresh.
Tweetbot and Twitteriffic don’t suffer from the Echofon sync annoyance I mentioned earlier, but it also doesn’t have the targeted notifications. Overall I think Echofon is the clear winner when it comes to syncing.

Conclusion:
If you haven’t already put money into a client or you use Twitter on your Mac a lot, you should probably get Echofon. Its UI leaves something to be desired, but its syncing is great and it is overall a nice client.
However, If you already have Tweetbot or Twitteriffic or you don’t like Echofon’s UI, you should probably just save yourself time and money by buying whichever you don’t have. That’s what I’m doing, at least until I get a MacBook and start using Twitter for OS X more.

You probably disagree with me for some reason or another, so you want to scream at me over the internet. You can reach me on Twitter, where I’m known as @Salax_.