I would suggest getting the evaluation versions of booth and kickin them around a bit.
I’ve used MM since its first or second version and ran a quick evaluation of strataframe for my shop this was pre 3.5 so I don’t know what StrataFrame did with WCF or WPF. Booth had some notable strengths and weakness. (to the flame police, keep in mind these are just my opinions and just my observations)

StrataFrame
Things I liked:
Outstanding support site (four full time developers and additional development staff as available). When I asked questions during the evaluation I got answers very quickly. I didn’t have a problem that didn’t get resolved that day.
Out standing documentation.
Several example applications.
Four developers working full time on the framework
A very polished look and feel from the designers to the example applications right down to the documentation.

Things I didn’t:
VERY heavily tied to SQL Server for project metadata.
The business object architecture was less granular then MM
Has some kind of bizarre online mechanism for switching activation if you develop on multiple workstations

Overall impression:
A very well thought out product with terrific support and documentation. The heavy metadata requirements and activation mechanism where show stoppers for us.

Mere Mortals
Things I like:
Business Object architecture is very flexible and easy to undersatnd.
Business Object architecture is very granular and divided into separate class’s
The Business Object Generator. Its gotten better in every version and offers a lot options.
Wizards and builders are straight forward and easy to use with no metadata storage requirement.
Windows form security model has been very easy to modify to our needs (single signon , encrypting connection strings…)

Things I don’t Like:
Mere Mortals is a one man show. If Kevin gets hit by a bus we are in trouble.
Example applications are god awful. There are to few , they don’t show enough of the framework, and they are unrealistic (see iPOD WPF example how about using the MM WPF forms).
Documentation, not enough of it, its not well organized and it lags behind the development its clearly an after thought.
Support. This is it. Check the times between posts and response it says it all. And if you don’t have a UT subscription you can’t search to see if someone already figured out your problem.
The 3.5 release , it went out the door before it was ready or tested it should have been labeled as Beta.


Overall
MM is the better choice in my opinion. Its business architecture is better and I found it to be flexible and straight forward. It lacks some of the polish I saw in strataframe but it gets the job done and its gotten better with every release.