Rather than playing around with PCP calculators I'd go and drive both, back to back. That will give you your answer, not everyone else's.
Well said.
As CraigW says too, coming from the rawness of a JCW and the silky smoothness of the 135i there are several things you'll be wanting to look at so you will be best advised to have a drive in each.
If you're buying for looks, which I'll assume you're not as you've had a 1 series, then a GTI is prettier.
More likely you're buying for driving dynamics, of which both have plenty, but the R nudges it in important areas.
Do you rarely take the car above 4000rpm? If not then the GTI might be a good bet as it's very torquey and both would be neck and neck up to around this sort of figure.
Above 4000 rpm the GTI is still responsive but feels a bit flat and tops out at around 5500. The R however comes alive at this point. It does feel a very different animal to drive when really pushing on.
If you're interested in fuel economy, which again I'll doubt you are (up to a point) having had a 135i, then the GTI is significantly better on fuel and runs on the cheaper stuff.
The R looks pretty plain in pictures but in the flesh when you get up close it's quite beefy at the front and at the back you have those darker lights and the piece de resistance are the 4 pipes. These do actually have a function (on the hatch) in that they make the car sound a lot better under load. If you turn down the electronic crap the car actually does sound ok. Nothing like as good as a JCW and even less so a BMW six but still passable.
Having said that, there's a GTI just around the corner from me and I can always tell when it goes past as I can hear it, which means it does sound ok and has a bit of volume as I hardly notice most of the cars going past.
Inside? Well if you're into black in a big way then the R will be heaven! The GTI has a more characterful interior if you go for cloth. The blue dials in the R are quite nice, but the 'piano black' plastic is a nightmare to keep clean. The golf ball gear knob is reason enough to buy a manual GTI on its own!
Handling? The PP GTI is very sharp and feels light on its feet (partly due the strong torque delivery) until you come to a very sharp bend when you are reminded of how heavy it actually is.
The R feels heavier again and is a real bruiser rather than lithe, but on the right roads it punches above its weight. Its traction and poise are very impressive but it lacks the GTIs nimbleness.
Traction? It's all about the R. The PP diff is very good so long as both wheels aren't pointing dead straight ahead. If the GTI does a straight ahead launch you get wheel spin as there's a lot of torque going through those front wheels. Do the same in an R and it does one of two things - get it a bit wrong and it bogs down; get it right and it just f**ks off up the road very very rapidly, no drama.
Hardly an exhaustive assessment but you get the picture.
I spent 20 months in a GTI PP and am just heading towards 12 months in a GTI.