Location for travel towards AMS, well if you're going by train, definitelly Kimpton, if by taxi, it's a tossup.

For a last minute sightseeing, as in, a quick walk after breakfast, I'd go with Kimpton, if you mean half a day and maybe a canal cruise, it makes for a little difference. I've stayed in the Hyatt Regency down the road from IC Amstel previously, and it was quite alright. As mentioned, a tram towards Leidseplein takes you to Rijksmuseum and canal cruise stops, as well as Heineken Experience is close by if that's your thing.

Regarding breakfast, you'd be able to get it around IC I think as well, but the choice near Kimpton is just massively better.
All things considered, I'd probably go for the Kimpton.