I am also considering buying a 700UZ. From the technical side, the main differences between the two models are:
The lens assembly - while the 2100 has a long lens assembly tube, the 700 has "telescopic" one. Means, the zoom actuator may choke some more energy out your batteries, but by optical means, the two lenses are the same
The 700 is somehow reduced in functional means - it lacks a picture stabilizer and an AF assist lamp. Well, it is controversial if a picture stabilizer is a must, since the camera is pretty light(I tried it in a local photo store). Also, there were postings at dpreview.com which suggested that digital picture stabilizing might be getting in the way when tracking a faster moving object.
The issue with the missing AF assist lamp means, autofocus can be not that optimal in low light conditions(especially at high zoom). However, you have always the option to choose manual focus instead which might be a help, even considered the LCD preview window. Using a tripod might help you too in such conditions, and since the camera is a real lightweight, it doesn't need an ultra-stable and expensive Manfrotto.