I'd be more concerned about the depreciation than the price difference. The actual cost is only determined when you sell it.
I've been watching prices for a while now since I've had the McLaren itch, and I don't think I'd want to buy a spider right away based on the depreciation of the couple. If you'e weighing whether or not it's worth $100K more, you should know that it could quite possible depreciate just as hard as the coupe within a year.
If you want a McLaren but ultimately want a spider, I'd just scoop up one of the cheaper 12C coupes you can find and see where the spiders settle after a year. That way you have a 12C to drive in the meantime and don't risk getting crushed with depreciation should it occur.
Personally, I think it will and I can't really see a reason why it wouldn't.