I'd find it hard to understand anyone who's used to a Golf even considering a Polo / A1 sized car (sporty variants or otherwise). They might be a bit of a hoot to drive...
... I doubt I would get tired of a car that was fun to drive. And the Golf is getting quite big. Some reviews have said that the Polo GTI is closer to the original GTI experience.
My experience is that small cars with big engines are very enjoyable to drive.
Our A3 was bought for comfort and economy, not performance. I love driving our A3, but experience has shown me that I would soon get bored of it.
Experience has also taught me that firm suspension and low profile tyres are not ideal for my local roads.
... this all brings me full circle back to another Golf, in my continuing quest to find the best car for me.
Mk5 1.9 TDI - very comfortable, but boring.
Mk6 GTD - enjoyable to drive, but quite firm - even with adjustable suspension.
Mk7.5 GTI PP with DCC - I'm prepared to accept the loss of economy and some comfort, for the gain in performance.
(then maybe a Mk8 Golf R, but a lot could happen between now and 2020)
My cousin's Fiesta ST is a very fun drive, but the interior is crap, and the Lupo GTI was a hoot - 125PS in a tiny car that weighed nowt (plenty of Aluminium used) back in 99, I wish i'd bought one rather than ragging my mate's at every opportunity and buying a boring 105PS Polo 1.9TDI myself instead.
My favourite Golf has always been the MK2 - The current Polo is pretty much equivalent to the MK2 in size. All cars are getting pretty bloated now - one of the reasons you have to go looking for the "Sheldon spot" when parked up in town, supermarkets etc. You used to have to be in a big car to get the comfort you'd need on a long journey, that is definitely no longer the case, and I don't do many long (>100 mile) journeys. The Golf is very well equipped compared to the Polo, and as you can see with the S1, by the time you spec it up to the S3, you might as well have bought an S3. I think I could definitely live with a Polo GTI if the next version exceeds the current one.