Firstly, thanks guys for all your help in making my purchase of a new MacBook pro so simple!
Now I'm moving on to my home machine- a iMac.
At home I would consider myself a 'light' user.
Usual stuff- watch videos, surfing, have a lot of music etc..
I am upgrading a 21" Late 2013 iMac.
Real estate- 21" is still the best size for me.
1st question-3.0GHz quad-core 7th-generation Intel or 3.4GHz quad-core 7th-generation Intel??
I'd go with the 3.4 GHz - not because of the higher clock speed on the processor, but for the Fusion Drive vs. standard HDD, and the better GPU. (Another option would be the 3.0GHz with a $100 Fusion upgrade.) Not sure the GPU will be that beneficial for your current use, but with things moving towards AR and VR, a better GPU may be beneficial down the road.
The standard configuration of the 3.0GHz is pretty close to what you have on the 2013 iMac (assuming you don't have Fusion on that 2013). If you don't have Fusion on the 2013, then it is time to step up to either Fusion or all-Flash. Besides the Retina display, that'd be the single-largest performance upgrade you can buy for this machine; far more important than either CPU speed or GPU.
If your next question was to be about Fusion vs. All-Flash... For your use, Fusion should be fine, even with the "small" 24GB of Flash that comes with the 1TB Fusion configuration. You'll have 1TB of internal storage running
everything at near-Flash speeds, with no need to manage fast vs. slow storage.
I'm assuming anyone moving from a previous iMac has at least 500GB already on the previous Mac's HDD, so spending $200 extra on 256GB Flash will require an additional $50-$75 investment in an external HDD that will always run at 20% the speed of the internal storage.
I have a late 2013 iMac with Fusion, and another late 2013 iMac with all-Flash. As I've said often in these forums, in everyday use it's very hard for me to detect a performance difference between those machines.