Lean is a nice package in 1.4 TSi FR (150) spec

We had the Leon in 1.4 TSi 140 PS spec and it was an absolute cracker. Ours had SE trim which lacked the sports seats and rubber band profile tyres which IMHO made it a better car. Unfortunately Seat in their wisdom decided at the mid life update that the SE should only be sold with the 130 PS engine and if you wanted the 150 PS motor you had to loose comfort and get the FR.

The other thing I would note is the throttle response difference between the early Mk3 cars and later cars, later cars had a momentary delay when pressing the loud pedal that early cars did not have. It is easily sorted, all you need is a chap with access to the VAG diagnostics and a simple "tick" in the Audi throttle response box sorts it. Seems only Audi drivers are allowed better driving manners now (same applies to VW and Skoda cars).

Not driven a Mazda 3 but the 6 we drove with the 2 litre engine was poor compared to the Leon, we bought a Skoda Superb 1.4 TSI 150 which was way better (even with the throttle response delay).

Mazda 3 for fun to drive, Seat Leon for practicality.

The Leon was certainly practical but it was also fun to drive. The ride and handling were excellent. Cannot see how the Mazda could be better especially when you consider it was not as quick.

Your specific car (140 SE) was a good combo of pace via the higher output engine (as opposed to the 125hp variant) and shod on sensible (but not unsporty) 16in tyres (more comfortable ride) rather than the 17-18in ones fitted to the FR spec cars, which may also have had more sporty suspension as well.

The Mazda3 was always a 'drivers car' whatever the performance (my 17yo 1.6 N/A petrol still handles very well), but I felt via a test drive was a bit dull in the performance stakes, needing to give it a lot of right foot to make progress. Very comfortable though as a 'daily driver, especially when paired to the very smooth TC auto box for use in heavier traffic.

If you avoid the Sport models, most come shod on sensible 16in tyres and the ride is fine, not so much on the 18in rims, a bit too firm for my liking except on lovely smooth roads.

It also has a high trim spec for the money, especially the SE-L (Nav), which comes with climate control as standard, whereas that's a cost option (when new) on the Leon SE, though standard on the FR.

What's a shame is that few Leon SE140s were offered for sale compared to the lower spec 125hp unit or the FR (in both engine specs or the later 150hp unit).

Especially for the 3dr 'SC' version, which a really lovely looking car (lie its Golf equivalent), something that 3rd gen Mazda3 didn't have an option for, although the Fastback (saloon) is a very nice looking car, just not as good on the practicality front with its small boot opening (but bigger boot space).

With our less than smooth roads, I'd advice the OP to do (as near as possible) back-to-back test drives over a decent distance and set of road/surface types that relate well to most of their driving pattern. I think that this along with the useability and practicality will be more important than sheer handling and performance, as most cars will easily do illegal speeds and go well round corners these days.

Cars that old will need a good history and look well-cared-for, and is prime importance. A once great drivers car that's not been well looked after won't be a good buy.

The OP might want to seek out a rarer Mazda3 2L 163PS uprated Sport unit - not that many around, same spec otherwise as the 122PS unit but remapped. Just don't expect a cosseted ride, and I wouldn't be surprised if it had been 'modded', which won't neceesarily be a good thing, especially for the insurance premium.