South Mountain, or the flattest road in the world?

Here's my weekend dilemma:

My employer sucks, so I have to work this Sunday. I have a 2.5 mile swim event on Saturday, so I have to do my long bike/run brick on Sunday. I don't know when I'll be done with work on Sunday, but we're usually done around 9-10AM. As soon as I'm done, I'm planning on leaving straight from work to one of these options:

South Mountain

or

A 4-mile bike lane loop in a small, quiet residential area

My plan is to do an 80 mile ride, followed by a run of 3-5 miles.

If I go to South Mountain, I'll do the TV Towers twice and San Juan road the rest of the time until I hit 80 miles. San Juan is great, since there's no cars allowed, but I don't really like going to the towers with cars driving up that narrow winding road. However, the scenery will be much better here, and it's a lot more challenging.

If I go to the little residential area, I'll be doing 20 laps in the same scenery, in the same gear, on the flattest of flat roads. I'll be done a lot sooner (which is good), but I might become mentally unstable. The longest I've gone on this road is 50 miles, and I was becoming suicidal at that point. On the bright side, this is all residential area with very little traffic and a big fat bicycle lane. I always feel very safe here, but I usually only ride in this area for shorter rides due to the intense boredom.

Keep in mind, I won't even be able to get started until about 10:30-11AM, and it will be pretty warm by then. Both of these places have their pros and cons, so I'm really torn on this one. And they're both about a 15 minute drive from work, although the residential ride is on the way home. If I wasn't working Sunday, I'd be at Fountain Hills on Sunday morning at 6AM. But that's not going to work for me this weekend.

Thoughts???