Best RainJacket out of these 2?

The Gabba isn't a waterproof in the traditional sense - it's a tight-fitting windproof with a high degree of weather resistance - ride it in a downpour and you'll still get wet, particularly as the seam aren't sealed taped. Importantly though, you'll stay warmer and you won't get the flappy nylon feeling from a traditional waterproof shell fabric.

On the otherhand, no shell fabric (regardless of the maker's claims) can handle the perspiration from a cyclist when working hard IME, therefore whilst the rain stays off, you'll get damp inside due to the water vapour from your sweat. If you want a shell, look at the Castelli or Sportful offerings which use proven Gore fabrics rather than a big pricetag and so-so materials of the Rapha.

Completely different pieces of kit.

You'd put the Rapha in your back pocket in case of rain. It would fill a pocket. Mine seems to.

The Gabba - you'd put on as your jersey if it was raining outside and you'd not take it off. Its too big for a pocket.

I've the short sleeve version - I'd go with that and the matching armwarmers - thats a more versatile combination.

I dont use my rapha rainjacket - its too nice to get messy - I prefer to use my Aldi cheapy jacket. Its a bit more flappy - doesnt look as cool - but 95% of the time it sits in my back pocket just in case - and it cost a fraction of the money - but it makes more sense if its raining - nice and fluo.

I did Lands End John O'Groats in 2011, mostly in the pissing rain. The Rapha rain jacket was a late impulse purchase and I was very glad I bought it. It performs very well. The only downside for me is that the cuffs can get a bit saturated if it rains for ages, but it packs very small and is very light.