When I first heard of the Shanshu Prophecy, my opinion was pretty much the same as what I assume most Bangel fans were thinking - yay!! Finally a way for them to be together, without any of Angel's break-up excuses. And of course, I was assuming it'd happen well within Buffy's lifetime.
Then I rewatched IWRY and that made me think. Wouldn't Angel turn down the Shanshu, if he could, like he eventually turned down being human in that episode? The reasons still stand, unfortunately.
I'll start by quoting Buffy herself, stating that she loved Angel more than she'd ever loved anyone in her life.
I'm also going to explain my preference by criticising her other pairings. Angel gets criticised for making some decisions on her behalf, but I think the others are worse.
Firstly, I've a problem with her being paired with any normal human. Angel broke up with her so that she could have a normal human life - but she isn't a normal human. She's a slayer. Realistically, she's not going to survive long enough to grow old with...