I don't pretend to be an expert, but I have been sailing since 2003, I have owned 3 boats (22, 28 and 35') and I have sailed on many more boats.
It seems like you are looking at typical production type racer / cruiser boats from the late 70's / early 80's. That can be confusing because there were a lot of manufacturers then, way more than even 10 years ago. Anyway, IMHO, most of the production fiberglass racer / cruisers from that era were pretty similar. This goes to appearance, accommodations, and sailing performance. Again, IMHO, there isn't that much difference between the 30' boat made by Catalina, Newport, O'day, Islander, S2, Hunter, Beneteau, Tartan, Sabre, C&C, Pearson, etc.
If you were looking for a heavy cruiser, something to go into the ocean or take on long cruises then things are different, same as if you were looking for a racer.
Back to the typical racer / cruiser, some boats will sail better than others, some will be more comfortable below, others will be roomier. The best thing for you to do is to look at lots of boats. You won't be able to sail on them, but you should find a few brokerages fairly close, see what 30'ish boats they have, and spend time on them. Then you will start to find out what features are important to you. For example, do you want a wheel or a tiller, deep keel or shoal draft, traveler on the cabin top, bridge deck, or near the helm, self tailing winches
or not, anchor
locker, swim platform, roller furling
, etc. On the inside, you want want / need an AC electrical system, refrigeration, hot water, shower, lots of storage?
What makes it real challenging is trying to compare different manufactures without seeing the specific boats. When new, I think everyone would agree that a Sabre 30 was 'better' than a Catalina 30. But, 30 years later, that same Sabre could have deteriorated into a piece of junk while the Catalina could have been well maintained and it now a much 'better' boat. So, don't get hung up on the brand so much as condition, cost, and location to you.