Re: For a family of 4 - Islander 28 vs Catalina 30
I think this is another one of those "well, it depends" questions. Only you can answer based on personal tastes and your priorities.
I have sailed both the Islander 28 and an '80s Catalina 30, and IMO the Islander is the better behaved sailor, although I don't remember it being particularly fast. I think there's more room and storage in the Catalina, although I could be wrong. I don't remember the I28 having an abundance of storage, but it wasn't something I'd look for on a day sail. And of course, this is anecdotal evidence, not objective and accurate statistics. YMMV.
If you want a somewhat more objective guide to speed, you might be interested in PHRF ratings. The '75 C30 w/o spinnaker (not tall rig) has a PHRF rating of 210, while the I28 w/o spinnaker has a PHRF rating of 224.
T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama