You never get disappointed with the sizes of the exhibits in safari parks - that's one edge they clearly have over traditional zoos. Safari parks also tend to have fairly good ungulate collections compared to traditional zoos.
Other than that, I prefer traditional zoos in most ways; you can choose your own pace (I guess you can do that in safari parks too, but if there are cars behind you, it'd be rude to randomly stop all the time), you don't need a car, you don't need someone to drive the car if you want to take pictures, traditional zoos often have bigger collections (especially when it comes to smaller animals), traditional zoos are more likely to have indoor viewing in case of bad weather etc.
We just had that thread about expensive zoo entry prices, and I find that safari parks tend to be comparatively more expensive than your average city zoo, even though safari parks often are "cheaper-looking" (i.e. many exhibits are just made by fencing off a piece of nature instead of actually building and decorating). I realize that maintaining a huge land area costs money, and that safari parks generally get fewer visitors, but that doesn't make them less expensive.
That said, I'm quite fond of the safari parks I've visited. But I'd generally choose some of the major city zoos I've visited over them.
