Long time lurker of this thread, now a satisfied owner of a 1363.
My Kernow 1363 arrived last week, however last summer I acquired Heljan's version cheaply in a sale. Or rather, I acquired 2, as the original Heljan expired after a mere 10 minutes running. The short answer: Kernow's model is miles ahead of Heljan's offering. I'll try and upload a photo comparison.
In terms of running, Kernow's 1363 initially ran with an audible sound, but following the prescribed running in period has quietened down considerably. As others have reported it is a very smooth running locomotive, capable of slow speeds. My (second) Heljan started out with a loud grating and grinding noise which has not reduced. It did not like slow speed use, constantly needing the 'hand of God' to encourage it to keep moving.
Appearance - at first glance, I'd say the KMRC version looks a lot better. Going into details I can only speak for 1363, but (with the exception of the much discussed splashers) I'd say Kernow's looks bang on the money. It's difficult for me to put into words, but seeing them side by side, Kernow's 1363 looks like an accurate replica/model of the specific locomotive, whereas the Heljan 1363 looks very 'generic'. I'd say it's a generic caricature of the entire 1361 class - something about the Heljan one doesn't look quite right.
The liveries are different too. Heljan's has a very olive-drab like colour to it, and the shirtbutton appears undersized. The motion has bizarrely been painted black. KMRC 1363's green looks authentic to the photo in post #1 for this topic, and the shirtbutton looks to be correctly sized. I'll try to upload a side-by-side photo.
I'm glad I pre-ordered the KMRC 1363, in my opinion it was well worth the wait. I'm not sure what to do with my Heljan model, although the thought of painting red stripes across the saddle, a large '6' on the bunker sides and a smiley face on the smokebox door does seem oddly appealing...