If you want apps to open fast, get it, with this caveat ... my 1TB fusion is fantastic for all software, but I have noticed that MS Word seems a tad sluggish compared to Photoshop, Illustrator, Firefox, Mail, etc. Keep in mind I do have 704 fonts active and when Word opens, it's the font reading that is stalling it. I measured ~4 seconds to fully open. In comparison Photoshop takes ~2 seconds with the same fonts active.
There are gazillions of posters here who will gladly tell you that you should upgrade to an SSD drive, no matter what the cost. The Fusion drive gives you (almost) the speed of SSD, combined with the size of a hard drive, at little cost.
If you start with the most basic Mac, and then ask what you might spend money on to improve it, the first item is _enough_ memory, the second item would be a Fusion drive (or SSD drive if you use little storage), and far behind comes a faster processor.