it has mainly to do with the traction (slip) curve of foam versus rubber tires, rather than durability. if you can get the steering out of a front diff on rubber tires, then by all means run it. diffs are easier on the transmission and reduce the front end chatter brought about by the cvd joint at extreme steering angles. however, most guys find it difficult to be fast (truly fast) with a standard front diff in the car.
a good compromise, right now, is running a front gear diff packed with between 100 and 500K oil. this will allow for the steering that most need, but offers some of the characteristics of a front diff. it's softer on center, but will roll more over the front end than a locked front axle (spool).
before you hock the spool, try getting a better setup with it. when the setup is good, the car will no longer feel like the front axle is pulling the rest of it around. doing this will give you a better feel for how a (gear) diff changes the car when you decide to go that way.
like anything, it's a matter of tradeoffs. rarely will you find a tuning aid that is all positive. set your car up around whichever you choose, and if the steering is there, the lap times should be as well.