What Mac best fits my needs?

Hi folks - I'm looking for some simple advice, and I cant see that anyone's answered these sort of questions for a couple of years now.

I think it's time to retire my iMac (late 2007, first of the brushed aluminium line). It's given me many happy years of service. I'm constantly changing my mind as to what I should get now. Either another iMac (21.5 inch) or perhaps a MacBook Pro (15 inch) - I have no strong preference in terms of mobility or a large screen.

My needs are thus: no heavy video or photo editing or graphic design, though basic stuff may become more common in a few years. What I do usually involves lots of spreadsheets, word docs, and browser tabs open at the same time, with the spreadsheets running reasonably heavy calculations (lots of vlookups etc.).

Whatever I get, it will have a SSD. Some people have told me this alone will make all the difference and worrying about CPUs, GPUs, and maxing out the RAM is pointless. I'm also opting for quad-core, as I often have multiple apps up.

So if my plan is to get the 21.5in iMac with the 2.7GHz quad-core i5, Iris Pro Graphics, 8GB memory, and 500GB SSD... will that still be meeting all my needs with good performance in six or seven years? If not, what should I upgrade? I can get the same machine with more memory, but can't upgrade the CPU without getting a separate GPU as well, in which case I'm happy to consider the basic 15 inch MacBook Pro with the better processor and memory but no separate graphics card.

Thanks!