I went from TT 1.8TFSI Sport to A3 1.4 COD - actually I have both so can directly compare. Both have S-Tronic boxes. The TT has a light grey interior which I much prefer but financially the only way to get all the toys with the A3 was to go S-Line which means black. Tech is much better on A3 - ACC is not even available on the latest Mark 3 TT. I love the Tech Pack on the A3 but now I have 7 digit postcode entry on the TT the RNS-E isn't so bad. As for driving, I enjoy both in their different ways - power is quite similar but the way it's delivered is quite different. Maybe because of the 19" wheels on the TT but it's definitely a more sporty drive, not harder but just more sporty. And the TT never ceases to amaze me how much stuff I can put into the boot, although the A3 is more practical. But driving the TT always puts a smile on my face! So I feel fortunate to be able to have both these cars and certainly you will not be disappointed with the A3. It is such a quiet and smooth engine, even more so than the TT which has a nice growl to it in the upper rev ranges.
Hope this helps.