I've had/have both and imho. these cars are not comparable in most ways.
My opinion the only things that are comparable: paint quality (lexus in fact is a bit worse) and rattles (the new Is is notorious for dash rattles).
If you do a lot of speedway miles then go for the lexus. The IS drives like a premium car but always feels a bit heavy and underpowered. IS-F or IS350 would be a better choice but the second will be hard to obtain. Stay away from the diesel (utter crap) and manual petrol.
Things that imho. should be avoided in a lexus: low spec. particular without the harman kardon setup, fabric especially the light interior (black or dark grey leather imho looks best), small wheels (the IS looks cheap with 16 or 17" alloys).
Biggest downside of an Is is that the rearseats don't fold so an IS wouldn't be very handy if it's your only car an you ocasionally want to ship something large that doesn't fit the ski compartment.
The Leon would be the better choice if it's your only ride and you want to have fun on B roads. The versions with bigger engines are fun to drive, feel agile and lighter than they really are.
Reliability is an issue with Seats and dealerships (at least over here) are way worse than those of toyota / lexus (a Lexus has way less problems to they will have more time for you).
The lexus in the end will cost you approx. 30-40% more than a comparable specced Seat so all will depend on your budget.
(my IS was Ice white with black leather, all specs without the sunroof and didn't have a single
problem for over 50.000 miles, My cupra I have so far had about eight minor and bigger issues on reliability but still is soo much fun to drive....)