I have both in TanDelta and I will try to provide a thorough compare and contrast. Their overall size is very similar, both in width and length.
Extra Force- This is the original ForceFin on steriods. Its bigger, faster, stiffer, and massively more powerful. I have been using this fin for about 10years now. I have always absolutely loved this fin, it can provide all the thrust that you can ever ask for. When you add the whiskers into the mix you can truly fine tune their power to precision sculling. The only issue with this fin, it takes some real leg power to propel and as such I think its probably too much fin for most people to use and I have only seen competitive swimmers have the kind of power and kick techinique to use them well.
Best uses- photography, open water where currents may be an issue, heavy gear loads where thrust is critical
If your dead set on buying this fin, I recommend that you get it in the Original Polyurethane vice TanDelta material.
Excellerating- This fin is all about its name, it accelerates. This fin has so much snap it unbelievable. It can flutter, frog, scull, back kick, or just about anything else that you can think of and does it well. I have never used any fin that can outperform this fin in anyway, shape or form. While it is not as fast as the Extra Force, it will get you to speed faster and easier. This fin although stiff compared to many, its not as stiff as the ExtraForce. It also has a much more traditional feel to it, as its recovery kick is much more pronounced than the Upturned styling of the prime line and the ExtraForce. This more balanced feel is why I believe that many people like this fin so much, as it allows you to have more control of your positioning by using the force of an upkick/recovery kick to move. Although, not as fast or powerful as the ExtraForce, it still has plenty to do what you want. I have been diving it in my Drysuit over the last couple of weeks and they do more than I could ask for. You'll see on the FF website, that Bob talks about clean edges cutting through the water. I can tell you this is completely true. The feel of these fins just flowing through the water with absolutely no sense of shutter, wobble, or twist, just plain and simple, clean flowing water. As a long time competitive swimmer, all I can say is, this fin must be how dolphins feel when they are riding a bow wave...incredible
Best uses- wreck/cave diving where positioning and silting may be an issue. General Scuba, I like this fin better than the ExtraForce, but its hard to nail down why, its just a feel thing. probably just that it is easier to kick for long periods and the clean feel of it in the water.
I know that you didn't ask but I will throw the TanDelta in as well.
Tan Delta- the original shape fin in the top end TanDelta material. This fin keeps all of the advantages of the prime line. I have just played with this fin for a couple of short dives, I have played with the Pros extensively. They feel very similar to the Pro/Orginal fins just a bit stiffer and more Go than the Pros. I really like these fins. They kick easily and they move extemely well. If you are looking for a Great Scuba fin, this is your fin. It can do whatever you want, they feel good and they kick wonderfully, I highly recommend this fin to pretty much anyone looking for the "one fin". This fin or the Pro is THE FIN that I recommend to anyone that is looking for a new scuba fin.
Best uses- all around scuba diving