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Compare prices between all 27" iMac models with the 2GB Fusion Drive. Middle model is 100 USD more, but you'll get a lot more powerful M390 GPU. Add another 100 USD and you'll get the best model with M395 and a bit better CPU.
The M395 model is what I'm looking at for the very reason you mentioned :)
 
Just got my BTO iMac last week, i figured the student/education discount i received, and an additional further 5% cashback for opening a new AMEX account about to end, seemed very worthwhile. I am truly in awe of this machine, all my nuanced reservations have been quenched. Wow, what a machine.

Was debating an ssd or fusion for some time. I have been using SSDs since 2009, and went all flash on my macs since 2010, thus a fusion felt like giant leap backwards. I wView attachment 602960 ent for the 500gb ssd, for my home and apps [have 300 gbs spare]. The speed of this hdd is appreciably quicker than my old sata 3 ssds, less so c.f. 2014 rMBP.

My data, music, movies, photos is stored on my qnap NAS, this also acts as a remote personal cloud.
Due to work commitments, I have yet to install windows on an external SSD via TB enclosure, found one cheapish on ebay a while back, and just grabbed a Samsung 840 500gb ssd 1/2 price.

SSD test:

upload_2015-11-30_6-57-32.png
 
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