kmayer- Good question about comparing polishes. I'll take the Griot's MP#4 as a perfect example:
The MP#4 is *AFAIK* just the MP#3 with some wax mixed in to further reduce the cut even further (correct me if I'm mistaken about that). So I'd pick a product that fills a similar niche- perhaps 1z Metallic Polish or their WaxPolishSoft. See if the 1Z a) does more correction while still finishing out well, b) leaves as good/better a finish, c) is more/less user-friendly, d) lasts longer if not topped with wax, and e) is more/less compatible with your last step product (LSP, i.e., "wax" or sealant).
FWIW I find polishes I like and then I pretty much stick with them. I'll *occasionally* try new stuff but not all that often and it's pretty rare for me to find something that really outperforms the stuff I've been using forever. The new scratch-resistant clears are a wildcard, requiring some different product technology, but unless those are an issue there just aren't all that many different things you need to have to get good results.
Some people enjoy playing around... uhm, I mean "experimenting with and broadening their knowledge base"

with a lot of different products though, and there sure are a lot of good ones on the market these days. There's an oft-repeated quote from a guy who worked in the autopaint division of PPG: "Find something you like and use it often". It's really about as simple as that.
Oh, and I'd expect to use different polishes (and perhaps different LSPs too) on your two Porsches. I have different regimens for my '80s cars compared with my more modern ones, not that the same stuff wouldn't be *OK* on both. You sorta stumble across combinations that *work* on certain cars. One of the benefits of this website is that somebody's almost certainly done a given car with a given product and you can consider their experience; you're not just flying blind.
If the '85 is single-stage paint (and isn't white), it'll probably be a lot softer than the clearcoat on the '03.