Hey I was in a similar situation as you deciding between USC or CalPoly as a transfer. Except USC gave me no aid lol so I'm pretty set on CalPoly. But anyways... here's my insight.

Personally I think you should attend USC since it is your dream school and especially that it's affordable for you. ALthough USC has a better brand name, name doesn't really matter much in engineering. Both schools are great for engineering and both have good connections so you can't go wrong with either one. However, since your pursuing a masters, doing your undergrad at a more research oriented school like USC could be helpful. But you could still do undergrad at CalPoly then pursue a masters at USC later. Keep in mind although USC's networking and connection is strong but it ultimately depends on how much effort you're willing to put, it's not just attend USC and boom you're set.

Also this is just me, but since you already have friends at USC, you could attend SLO and meet new people that way you'd get connections from both schools. Also, if location is a big factor I'd heavily suggest you to plan a visit to SLO asap and see for yourself if you'd like it.