While the W is awesome, my friend and I agreed that we would have been disappointed to only do the W. It only takes a few more days but is oh so worth it!!!
If you do end up only doing the W, make sure you make it up to the camp/Refugio at Grey
Go as far up the French valley as you can
No matter how tired you are, get up before dawn and hike to the Torres to watch the sunrise. It was epic.
I have a trip report posted at backpackerPT.com
I would also recommend the singing lamb hostel in puerto Natales. It was a most perfect place to stay before and after the trek.
Chile is way more expensive then you think.
Make sure you don't think of it as a wilderness trip but rather an "international trek." Since you don't have much experience with international travel and "trekking," just watch your expectations...it is a very, very different experience.
The campgrounds are crowded (on the W...not so much the backside of the circuit), the toilets are sometimes really gross, but people are super friendly, the hiking is pretty easy but literally mind blowing, and all in all it's a trip of a lifetime.
Feel free to PM for more specific questions...I love talking about my trips :)