Here's a brief comparison:
Out of the box, Manila lets you build certain types of sites easily, but not all types of site. I was using it on my resume web site, but I ran into many problems with static rendering (FTP export) and gave up and switched to Front Page (ugh). ETP has no concept of static rendering - all the pages are dynamic. That's probably the biggest difference from an out of box perspective: Manila works with cheap hosting resources via FTP, ETP requires you to install and maintain the full OpenACS stack. Manila server, OTOH, has the same requirements - a server.
Manila's templating system doesn't require you to have file-system access to the server, ETP's does.
Another minor issue: Manila has no concept of versioning - ETP distinguishes between 'live' revisions and prior revisions. In manila, everything just gets served out of the object database.
Both toolkits allow you to quickly get a site into production built by a team of users. ETP provides much more of a foundation to go beyond that simple application. Manila starts and stops at it's out of the box experience. I really enjoy working with Frontier, and in theory you can customize it, but in practice the paradigm is tough to grok, and the dev tools are nonexistent.
