[ExpertWP]

You can use both for Web design depends upon your choice but when it comes to deliver web designs/wire frames early then Illustrator is a lot quicker and easier to use for this task.
I want to add that, for many designers (including myself) Photoshop is usually the first choice here. As Photoshop graphics sit on a pixel based bitmap it would seem like the only choice for designing on screen media.

However, when designing user interfaces, Illustrator can provide many advantages that Photoshop cannot. Firstly, using Illustrator here makes the job a lot faster - there's the obvious resizing points. Illustrator is also great for creating reusable components. Using the symbols panel in Illustrator, you can create a library of icons and form elements that can be reused and manipulated many times over.