ICOs have been a sort of get-rich-quick scheme lately, and it's very likely that the price will spike and plummet like you say. If you're investing for the long run, I'd put 75% into the ICO and then hold out to pick up some cheaper tokens after a dip. Still, I'd put most of my investment in upfront since there will be the greatest support at that amount. Personally, I'll be doing 100% at the ICO.