I have both and apart from the merino ones having shrunk a little in the wash, the rapha ones also keep the wind out better. They are not the merino type but I actually prefer the rapha ones. Just wish they did a plain black design.
Funnily enough I have the törm version of those (non-merino) and I prefer the PX merino precisely because it lets more air through, whilst still remaining warm.
Can they be 10x more amazing than the £3.99 planet x merino armwarmers though?