Let's get to the bottom line first...choosing a Catrike 700 or Catrike Expedition is a matter of personal choice. Both the 700 and the Expedition are solid trikes, built to handle tough riding and backed up by Catrike costumer service. The 700 and the Expedition are both stable at speed and share the same handling characteristics.
Catrike 700 / 20 (new for 2013, 700 with 20" front wheels)
The 700 is an awesome trike, it looks fast and is fast. It's a ride best suited for getting an adrenalin fix. The 700 should be kept light and geared for speed on the flats and downhill runs. Grinding up long roads with extreme grades takes some effort, but then there is the sweet downhill. Exceeding 40mph on a downhill is par for 700. The fun comes trying to find a road that is long, steep, reasonably straight and has good pavement so the 700 can show it's stuff going over 50mph. I do suggest when exceeding 50mph that you are acquainted with the road so there are no surprises and wear a helmet! On the flats I generally pass more DF (diamond frame) riders then pass me. In recent memory I don't recall ever being passed on a downhill. Even the young, fit and properly attired lads with advertising plastered over their lyra bodies fade away in my rear view mirror. Riding a 700 is a prescription for an old fart like me to feel the thrill of an endorphin induced high.
That endorphin high comes at a price. The 700, when set up for speed, is a hard ride that given the conditions of some our streets it can shake your fillings out. On long rides the seat being laid back at 25 degrees may be uncomfortable for some folks on long rides. I don't consider it a touring trike. The 23C tires (if you want speed) are going to get flats. The skinny 23C's trade puncture protection for performance. Of course some of these setbacks can be mitigated with different tire selection and putting a rack on (which at did at one time) to carry more stuff.
Ahh my faithful and trusted ride. The Expedition is a versatile ride. It can be setup to ride fast or loaded down to tour the world in comfort laid back in it's 37 degree reclined seat. Keeping the the Expedition light and adding high performance tires would bring the Expedition close to matching the the performance of the 700. I prefer to ride in comfort when not screaming down a hill on the 700. I've loaded my Expedition up with Marathon Plus tires all around, heavy duty rack, rack bag and panniers, extra tools and other goodies that give me confidence that I can survive the apocalypse with nothing but my Catrike Expedition.
Another benefit, for me personally, I'm inclined to enjoy the world around me more as I meander with Betzi in tow and on a solo ride. The 700 makes me feel like I have to ride hard and fast whereas the Expedition is telling me to take it easy and smell the roses. Trike, body and mind are about 20% slower when riding the Expedition compared to the 700. Also the Expedition is geared for climbing and not speed. But even on this beast I still pass my share of DFers and that's a fun blog post for another time.
The only addition which would be interesting to see is a fully suspended Expedition so it's versatility can be enhanced by giving it the ability to go off road with comfort.