It sounds like a winner for DX. Essentially giving you the 70-200 range for DX and keeping it fast AND sharp. Not sure how I feel about the price. If I knew that I would stay DX then I wouldn't think twice. But if I felt that I would eventually go to FX then I'd have to weigh my options (when am I going to FX, will I do both DX and FX, etc.)
The place you want to order from, do they allow for a 30 day return? If so I'd do it for sure. It might cost you about $30 to ship it back but I think that's a fair price to get hands on knowledge.