Yes, an SSD is significantly faster than the mechanical drives we've grown accustomed to. In fact, these days...it's the NUMBER ONE improvement upgrade you can do to ANY system, IMHO. RAM, CPU...forget about it. An SSD will provide immediate and noticable speed improvements for just about anything you do that relies on accessing data.
That said, regardless of which choice you make, you'll want a second drive for your footage anyway. If your OS and editing software lives on your local (internal) drive...grab a nice FW800 external and use that for your media.
The new i5/i7 MBPs are more than fine for editing with FCP. Very portable as well...I make my mortgage doing video and audio production, and just picked up the 17" i5. I'm actually going to apply the couple hundred saved on the jump to an i7 for an SSD upgrade...maybe 6 months or so down the road, for an internal drive...as the prices come down a wee bit more.
With the iMac, you get more screen real estate...but you lose portability. Either choice is going to be pretty comparable when it comes to speed. There's a lot of crossover on the amount of RAM and CPU choices between the two lines...however, the high end 27" iMac DOES provide for the option to have two drives internally...one being an SSD. That's a cool bonus.
If it's portability you need, don't worry about performance issues with the new line of MBPs. They're fast computers relative to the software on the market...even the Adobe products. I also run Premier/AE/PS and On Location...All Good!
J