Having worked extensively with FI, I can tell you that there is no reason other than if the injector or chip is visibly broke that I wouldn't use it again. There is a big difference between and oil pump and a ECM chip or Injector. For one, if the injector or chip isn't functioning, you are going to know pretty quickly, and two, they are easily accessible.
If you are looking for someone to tell you that rebuidling the 305 is the best option, then that will have to come from someone else. With a little work and research, you can have a fresh TBI 350 in your 88.
If you just want a decent running engine in your 88 without much concern for higher HP potential, then run the 305 and dont even hassle with the 89's engine.
For the most part, the chip between your two TBI's is the same. Other than a different fuel map and spark curve they contain mainly the same pre-programming. Your 350 would even run off the 305's chip, though it might run a little lean.
You said it yourself, no replacement for displacement. It just depends on what you want out of this.