Both are somewhat out of the downtown shopping "core" but the Pan Pacific is closer to the business core. However, it's very easy to take the Skytrain (subway) from the Pan Pacific.

The Westin is a much newer hotel than the Pan Pacific but not connected via Skytrain and pretty much out of the way for most things.

Might I suggest the Fairmont Waterfront (right across from the Pan Pacific). It was recently rated #12 in N Amer from the Conde Nast list.