There's nothing wrong with alternating different fertilizers. Look at it this way - say you spend $10 on one and use it up in 3 months. Or, you can spend $10 on two different fertilizers, alternate them, and in 3 months you'll have used half of each - still $10. So cost is not a factor, unless one is more expensive than the other.
Many recommend alternating fertilizers, as different fertilizers have somewhat different mixtures of trace elements. Is it necessary? Probably not, especially if your trees are healthy. But a fertilizer "regimen" can just as easily include alternating 2 or more different types, as using one exclusively.
I have 2 that I alternate, one is a standard miracle-gro type (contains a fairly high amount of urea), the other is a more specialty type that contains little or no urea. I do tend to use the urea-free formula more early and late in the season, when soil bacteria may not be as active (urea requires bacterial action to break it down to components usable by the plant...more effective when it is warm).
I also add organic ferts to some trees. I don't really like the timed-release ferts except for stock plants.
