I've used a Terry saddle for a while now that has a cut out. I've had to replace it at about 4,000 mile mark every time. I could tell when it was time because I would suddenly start to have chafing and discomfort. I talked to Susan at TE about it once and she reported the same experience. Cutout saddles often break down more quickly than others, and it will vary from brand to brand and model to model. It sounds like you've gotten a good life out of your current saddle, but unless it's hurting you or you otherwise don't feel it's supporting you, then I wouldn't necessarily feel compelled to replace it.

As for the difference in Terry saddles, I can't say for sure. I would presume that gel saddles offer more cushion in targeted areas, which may or may not be a good thing for all riders. I would also assume that the more cushion a saddle has, the more quickly it will break down as that cushion becomes compressed over time. My Terry saddle of choice--the Butterfly Tri Gel--is no longer in production. It's time I find a new model I guess.