The 1.5 is only a tiny bit more efficient and, I believe, has very slightly more torque. There aren't any major differences between them, the design has been tweaked and improved, but not greatly.
You won't notice the difference in real life, I drove both engines and couldn't tell, but only the 1.4 was available in the Leon at the time.
If I were repeating my purchase I'd probably go for the 1.5, just because it's 'better', but the DSG instead of the manual I chose. I'm not impressed with the grabby clutch on the Leon, after my previous TDI, nor the very short first and second gear. The extra maintenance costs for the DSG have a bearing though.