Another thing to think about, with the OP being from Canada and all, is the cost of airfare. Flying into and out of Palm Springs airport is expensive, even for us folks here in the good ol' US of A. The cheapest airport with relatively easy driving access to Rancho Mirage is in Ontario, California, which is about a 70 minute drive to WHM.
A good 30 minutes of that 70 minute drive from ONT to WMH is in good, ol' outskirts of LA, freeway driving (where it is apparently against the law to signal lane changes, let anyone in, or drive at approximately the same speed as anyone else.)
Phoenix has heavy traffic too at times, but it just seems so much more civilized than LA traffic, and WKV is way closer to PHX than WMH is to ONT.
Also, with the Westin Desert Willow coming online in Palm Desert, WMH risks becoming the weak sister in that area (like SDO is to WKV in Scottsdale, and SMV will be to WRV in Avon).
In summary, both resorts (WMH and WKV) are very nice, but the airfare/freeway traffic issue and having access to the lazy river at that adjacent Westin Kierland hotel swings my vote to WKV.
-nodge
Oh yeah, the waterslide at WMH's main timeshare pool is way, way, way better than the slide at WKV or that meandering essentially slope-free thing they call a slide at the Westin Kierland hotel. So if you are into waterslides, WMH's is the way to go. Then again, if you are into waterslides, Harborside beats 'em all, hands down.