Either of the two options you cite would do the basic job, but both have issues.
I haven't evaluated PeakSimple, but from looking at the SRI web site, it looks like a fairly basic package that may not support all the GC data processing, data reporting, and data management capabilities you may need.
The AI-450 software has been discontinued for about a decade, so you won't be seeing any new releases of that -- if someone's trying to sell you a used package for $2K, I suggest that you shop elsewhere! (By way of full disclosure, I was the product manager for it.)
Your posting cites only two options. Most CDS vendors offer 5890 control. I'd suggest that you also consider Chromeleon (the modern Dionex CDS), and ChemStation and EZChrom from Agilent -- these seem to be the most popular ones used with Agilent GCs these days.
It's always a good idea to test-drive the products before buying, if possible. Ultimately, what's best for you will depend on your needs and your budget.