Jim,
We just contemplated that exact same decision. What do you get for 2 feet and either 10k or 20k depending upon sedan vs Flybridge. You get, the improved cockpit as previously mentioned, you get an extra head and you get an engine that is completely outside the interior cabin. Other than the obvious flying bridge - that's pretty much it! For us, we expect to have a lot of visitors so a cockpit that is more serviceable was a desire. My spouse was also very concerned about visitors needing to enter the "master cabin" in order to use the head and our hope is to be able to force the use of the aft head (except for showers of course). Although there is a fwd curtain you can pull in the master cabin, it is not the same as a real door. Those two capabilities drove the purchase of the R-31. It was worth it for us but may not be justification for others. From 25EC through 31FB, it is all about what an additional feature/capability is worth to the user/purchaser. The 25 is probably the best economical value from that perspective. We purchased a R-31FB.