Traciatim :
It's doubtful they would even be able to notice the difference.
An SSD, SSD Cache, or Hybrid Drive would probably be far more noticeable for day to day tasks since in general use the processor mostly sits around idle. Yet when you click to open your browser and e-mail and they are on the screen faster than you can move your hand to your keyboard to type the URL or E-mail it makes your machine feel great and responsive.
Yes, maybe in general use, the SSD would be the best upgrade. The Pentium does have a bit of grunt, although not nearly as much as the i3.
However, I want to give an anecdotal (take it for what you paid for it) experience that I just suffered through yesterday. I was working on a HP 2000 Notebook (AMD E1-1200 APU). I was loading the Windows 8.1 update on it, and it literally took close to two hours for it to download (a 75Mbps Internet connection, so it wasn't the Internet speed) & then apply the update. I can't tell you how many times the "Almost there" and the other nice text things along with the color gradient fills cycled through. Then I had to download and apply another 100MB of updates - there went another hour of my life.
So while it might be fine for small things like reading email and maybe light web browsing, it was a dead dog when it came to installing programs. It is a travesty that HP even sold that extremely under-powered Zacate platform, and I'll never get the hours back that were spent just waiting for it to load. I considered advising the owner to buy a SSD for it, but then I realized it still had the same underpowered CPU (APU platform - I'm sure the Kabini platform is much better). So I recommended replacing the whole notebook itself.
So, for me, I'd take the i3 myself first, and then consider a SSD later. The Haswell Pentium isn't bad though, so it won't be near the terrible experience I had. You could even go with the i3 and a small SSD if the end-user won't be storing tons of pictures/videos, like the Crucial MX100 - 128GB for $75 from Newegg.