S4 is my least favorite of Buffy (by quite a bit actually), but The Freshman is probably my favorite season opener after the series premier. It's a fantastic episode. The Angel S4 opener was solid.
You know, I actually think S4 of Angel is quite underrated. The overall arc with Jasmine is just dumb (that's about the best word I can think of to describe it) but beyond her and how off the rails it gets during that arc, I really enjoyed S4, especially after re-watching it. There are a lot of fun episodes and while Connor is freaking annoying during most of it, he doesn't ruin the season like I had remembered him doing. I'm also one of the few who actually liked the storyline with The Beast and Angelus, and the Faith return was awesome.
Which had the better arc? I think the potential was there in Buffy S4 to have a really cool storyline. Having the government and the military get involved fixed a huge plot hole from the first few seasons and opened up what I thought were going to be a wide range of interesting possibilities. I just don't think it worked out. Adam was boring, I found it just impossible to get into Riley and the whole Initiative thing got really bogged down at times. There are great stand alone episodes in S4 and it's not that I hated it or anything, but I think it fell short of what it could have been.
If we're talking arcs, I also thought that in the first three seasons of Buffy, the show really, truly understood high school, and absolutely nailed everything about it. They didn't really capture the same magic with college.
I'd say it's a bit of a coin flip. The Angel S4 arc is great all the way up until the end. The climax of Jasmine was a total letdown, but the rest of the season was great, so I'd probably have to choose Angel.