Caribbean - Grenada vs. St. Lucia

I'm planning a trip for two for sometime in March or April, and so far I've been most interested in these options. The main things we're looking for are:

Beautiful beaches
No tourist trap feel -- or at least the ability to get away from touristy areas relatively easily
Variety of activity -- we don't just want to sit on the beach for a week.
ZIp-lining, preferably
Rainforest or similar scenic beauty on the island

And ideally, we're trying to keep the cost in the $2500-$4000 range, including airfare (coming from Philly or Newark).

Am I on the right track looking at Grenada and St. Lucia? Which would better fit what we're looking for? Are there any other islands I should be seriously considering?

Thanks!

I have been traveling to both islands on and off between actual visits and cruise ship calls for the last 40 years. Both are incredibly lush and green, very South Pacific like.

That said, I much prefer Grenada. The town of St George's is incredibly picturesque, built around a "horseshoe" harbour, like a mini San Francisco with the homes running up the hillsides. The beaches in Grenada like Grand Anse are far whiter than those on St. Lucia. And the people in Grenada are far more gentile and polite than those on St. Lucia.

Just my two cents!

Worldspan

A friend of the family found the St Lucians very rude - I was a bit surprised by that as I had not heard that before, and was considering going - do you think that was a fair comment?

To answer the OP, the other island which fits reasonably well is Tobago. It's very un-commercialised, a fairly mountainous island complete with rainforest and rainforest hiking and some beautiful beaches.