What is the computer for? Gaming, Design, Rendering, General PC, etc? Honestly there isn't a large different between a mobile i7 and a desktop i7 now (as long as they are at the same clock/speed) so I don't think a Desktop i3 will really beat the i7-4785T in the All-in-One. Which i3 are you talking about?
The i7 4785T is a 2.2GHz Quad-Core w/ Hyper Threading that will boost up to 3.2GHz. My guess is it will probably be able to boost up to 2.8/2.9GHz on all 4-cores, basically making it a 2.8/2.9GHz Haswell i7. I feel like when you use programs that uses more than two cores the i7 4785T will be much better.
This all depends on what kind of i3 you get. The i7-4785T will most likely run at 3.1-3.2GHz on two cores so it can be a 3.1-3.2GHz i3 or a 2.8-2.9GHz i7. The i7-4785T also has 8MB of Cache instead of 4MB on the i3. Not sure how much of performance difference that will make.