Welcome aboard! Looking at your other posts it looks like you either have done lots of research or have had rats before.
I love learning new things so I'm glad of this post.
In terms of comparisons for me, from everything I've seen with guinea pigs, they're pretty mellow and tolerant as far as rodents and rodentish type pets go. Pigs need a flat non-wire/solid, level, large floor surface and don't like having their feet off the ground. They're also quite vocal. They are social and get on well with other piggies, but don't bond as highly with their people as other critters. They live 2-6 years longer than rats.
Rats in general explore and climb and are problem-solvingly smart thanks to their predator make up. This makes them very entertaining, but it also requires more effort in terms of 'rat-proofing' their surroundings. Rats will only live up to 2-4 years max, most living 18 mos-2 1/2 years. Rats are social like piggies, and should be kept with same sex friends or will become depressed or neurotic. Rats will often bond very strongly with other rats and their people, and keeping other rats won't make them less social to people. Their social needs are much more complex than Piggies.
Rats require more interaction and time from their people, but are very quiet and clean. They don't really need bathing unless they get into something nasty, and they don't require brushing unlike piggies. A healthy diet, periodic nail clipping (as with piggies), and a little olive oil in their diet is pretty much all they regularly need for grooming. If you must, you can wipe them down with a damp cloth periodically and I do very occasionally bathe mine in kitten shampoo if they just happen to get befouled and I don't want to wait for them to clean themselves, but fewer shampoo baths are better.
Kept equally, for me personally, I prefer the scent of rats. It's not as noxious to me as piggy cages. Though this may be just a personal preference, it seems that piggies just smell 'sharper' and more 'barn-yardy' to me.
To me, a dirty piggy cage smells of a reeking barnyard. A dirty rat cage smells of a stuffy elementary school classroom right after PE. Neither are pleasant, but I can handle a stuffy elementary room much better. LOL
Rats can be kept in a narrower floor space, since they actively prefer space that provides climbing opportunities. So you can build higher up instead of broader out, but I've seen that it can be much cheaper and less complex to make a proper cage for a pig.
Similarities: Cost wise, it may be cheaper to purchase a rat the first day, but with the cost of proper caging, diet, and health care, rats are every bit as expensive to keep properly as piggies over time.
Both rats and piggies have specific dietary needs that need to be carefully accounted for.
Hope this helps.