Both the 3 Series and the Mondeo will have better steering than the A5. The A5 is fairly bland to drive - it isn't a drivers car, it doesn't really reward enthusiastic driving. It's really just a posh but ordinary car. Which is absolutely fine - infact for many people thats exactly what they want and it's why the A5 is so popular. If you are not bothered about communicative steering, excellent handling and all that jazz there is nothing wrong with an A5.
However consider whether a nearly new one is sensible. The A5 is a very old car now, it first appeared almost 8 years ago as a competitor to the long discontinued 3 Series. This means that, the odd cosmetic facelift aside, it's essentially the same car as it was in 2007. Therefore I'd question really whether they are still worth £25k+. As a result, I would personally either:
a) Look at a brand new A4 instead of a nearly new A5. It's a completely fresh car on a fresh platform and is thoroughly more modern in every single way
b) Buy an older, much cheaper, A5 Sportback.