All will work but I've had most performance and stability with using JQuery and a script service.
An update panel will work but as its designed to work in the general case it can be a little bloated if you aren't careful. One reason being is that it posts the viewstate back with each update.
I recommend you check out the site: encosia.com as it has a number of posts on how to use script services and page methods. Then you can make a more informed decision. I would say that I've not used page methods and probably won't but that's entirely for personal reasons.
One other thing to consider will be any mvc you may end up doing. The general consensus is that it's a whole lot easier with JQuery. Added to that Microsoft have officially adopted it and also provide intellisense in VS2008.
All compelling reasons to include at least some of it in your project. In mine I only use the built in ScriptManager when I absolutely have to.
HTH!
S