I like the Leon's.
For standard models, I think the Leon is much better looking that the Golf. I like how it keeps a good useful shape whilst still being very stylish (the Astra 3 door for example looks good but isn't as practical).
For performance models I prefer the subtlety of the GTI over the Cupra models. The K1 is great looking both in pictures and on the road but it's just a bit aggressive for my own taste.
Interior wise I'd say the Leon falls a bit short of the Golf but I have to say, it's hardly something to complain about. Having been in a friends K1 it's not exactly a bad place to be. I actually quite like the Leon interior. It's just that I prefer the Golf's.
To be honest, of the two cars you have suggested, I'd go for the Seat. It's a bit more unusual (most FR's are diesel), it's a bit younger (age/less miles) and it's cheaper for insurance. The saving in cost is also a bonus.
Just don't put a K1 kit on it. Leave that for the folks that actually have the K1.