If you are dead set on GSAPP, I would not substitute out 1 of the 3 years for a year at Pratt. You want the education and experience of GSAPP or do you crave the name? Don't treat grad school like undergrad (community college, then move to the "big name" school). Grad school is an expensive commitment and if you have analyzed the things that you want to achieve/learn/experience and those things are more likely to occur at one place over another...don't dilute (delude) yourself. 

I'm not sure about the transfer process, but I would imagine it's not straightforward and you'd lose time adjusting. 

Also, don't compare yourself to people that willingly post their work on this forum or others. It's like instagram in that you are only seeing the best (or worst) of a very small slice of the population. You'll end up over-valuing the style and content of such portfolios, to the detriment of your own. If you have a portfolio worthy of grad school, then submit to the grad school that you'd like to attend. Genuine interest and aspiration trumps a dolled-up portfolio any day.

Yes, transferring from Pratt (or anywhere) will decrease your chances of getting in. The number of people accepted as transfers is much smaller than the number accepted directly into first year - and the number of transfer spots available depends on other students dropping out or taking leaves of absence.  That's unpredictable at best, and some years there have been no spots at all when enrollment has happened to be higher than projected. 

Your chances are almost certainly stronger if you apply as a first-year without an architecture background than if you try to transfer after a year elsewhere.  As someone without an architecture background your portfolio just needs to demonstrate original thinking and some design process, and strong interest in some type of visual arts (doesn't have to be directly architecture-related).  But as a transfer your portfolio needs to demonstrate full mastery of first-year-level M.Arch work - and it would have to be way above average to get a transfer spot - and you'd have to somehow have found time to complete that portfolio of first-year work when you're actually only partway into your academic year, and while excelling in your work at Pratt.  All of that would make it an unlikely feat.

Really helpful! Thank you both for your thoughtful responses :-)

You should also understand that there isn't really such a thing as a "transfer" into GSAPP's M.Arch.  If you spend a year somewhere else, then what you'd be doing is applying as a new M.Arch student, but as one with advanced standing.  The % admitted by that route is indeed extremely low, and sometimes zero - and on top of that you won't necessarily get transfer credit or a waiver for everything that you took in another university's M.Arch program.  Sometimes it's possible to switch from one university to another and not end up spending more time overall on your graduate degree - but a lot of times it's not.  Transferring is much less common at the graduate level than it is for undergrads, and graduate programs aren't as used to dealing with that, or as invested in making it go smoothly.