from a stock stand point the two have barely any substantial gains over one another. both hit 62mph in the 13 secs,both top out at 100 and straight away.
the two things I see noticable is the tq the 1.3 has over the 1.4 plus the mpg and the per lt price of there fuels.
another thing you should note is the difference you can gain with aftermarket parts. with an intake,full exhaust system,remap youd be lucky to see a 10hp difference on the 1.4 whereas the 1.3 would easily add another 30hp not to mention a **** load of torque too!
the guys above mentioned about a turbo lag but Iam almost certain they all have been talking about the 75 hp conventional turbo version while the 90hp 1.3(11.5 secs to 62mph,a big 2 sec difference) version got the variable geometry turbo with little to no lag at all and that could be made even better with the result of some modifications too.
conclusion= no matter what point you look at it the 1.3 in my own personal opinion is better than the 1.4 which is why I my self picked one!
I have a friend that has added an cold air intake,downpipe,decat,b-pipe,muffler,a bigger variable turbo from another car and a remap who has brought his 90hp 1.3 to 125hp I would like to see a 1.4 get that.