not sure i agree, although maybe its because i heard them first.
I think the songwriting is probably stronger on those two, although the arrangements are not quite up to the same standard.
This isn't necessarily a bad thing though, illinoise is a little overblow and shiny in parts and sufjan is at his best stripped down - just him and his banjo!
Sister is probably my favourite sufjan song.. perhaps...
after all that though, illinoise probably is his best! (just)
edit:
just to add to this, listening to seven swans now, i think the songs mean more to me after seeing him live in a small venue in 2003, there was a little story between each song, and you got a whole backstory for each one... he has the best banter (it was complete with OHP projector with slides of michigan)