If those are your choices, I'd choose Grenada over St. Lucia. It's been a while since I've been to either, but I found Grenadians to be among the warmest, kindest, and friendliest of all the people I've encountered in the Caribbean.

Topographically, there are some similarities between the islands -- lush vegetation, high hills, geological features. Beyond that, Grenada has some gorgeous beaches (natural ones), including white sand Grand Anse, which you don't necessarily expect for a volcanic island. Also, Grenada grows a lot of spices and some of its own food, which lends itself to a unique local cuisine.

There are lots of unique local accommodations on the island. The best beaches are on the L'anse Aux Epines (Prickly Bay) peninsula around through Grand Anse, so if beaches are important, that's where you might want to look. I'll also point you to La Sagesse, which is on the Atlantic coast; it's a grey sand beach which will make you think of the South Pacific, and there is a modest inn there.