So i've been pretty much set on getting the 21.5 iMac with the following specs;
3.1ghz (Turbo to 3.9) i7 processor
16GB Ram
1TB Fusion Drive
NVIDIA GeForce GT 750M 1GB
27" model...
3.4GHz (Turbo to 3.8GHz ) i5 Processor
8GB Ram (kept to a minimum as its easy and cheaper to install extra on the 27" model)
3TB Fusion Drive
NVIDIA GeForce GTX 775M 2GB
...
CPU: either will be more than adequate for your needs. the i5 lacks hyperthreading, but you won't miss it for what you'll be using the computer for.
RAM: 8GB will be enough for your needs for the next few years... and if you get the 27", you can always add more later.
SSD/Fusion Drive: I'm a proponent of pure SSD myself, and 256GB is plenty for basic usage. You're only going to run out if you amass a huge video or photo library... in which case you get an external drive for that. Eliminating the internal hard drive component will also reduce noise and heat. However, if that criteria doesn't meet your needs, a Fusion drive is a good compromise.
GPU: Again, either will be fine for your needs. None of your usage will benefit noticeably from a faster GPU. You didn't mention "video games", but that will obviously benefit from the higher end card.
Screen: The 27" wins hands down - more screen real estate, and while the new 5K retina screen is amazing, the non-retina is no slouch.
Conclusion: There's no reason not to get the 27" iMac unless it won't fit on your desk or won't fit in your budget.
Enjoy the new computer when you get it!
I am pretty sure most people just compare their new SSD iMac to their say 2007/8/9 iMac with a HDD and say the HDD is so slow in comparison. The HDD is not slow in comprision, it's their 5+ year old iMac is slow in comparison to their new one.
Martin, I respect that everyone's entitled to share their "personal experience", but you're so wrong on this SSD thing that it hurts. I would suggest keeping this one to yourself, because you'll only invite ridicule from experienced users and confuse newer users. But I'm glad the HDD is working out for you!