Trick subject, it is apples to pears.
We discussed with Aidan about RTS vs FPS games in terms of what is easier. He argued that RTS is very time consuming and he could pick up any FPS. I would agree, but I won't.
Though, in general FPS skills transfer over from games easier as they're mainly reflex/aim skills. For example, if you're good at Quake you're probably gonna be good at Unreal Tournament, Team Fortress etc. (Once you learn the basics obviously). Those good at TFC generally good at TF2 for example.
However, RTS skills aren't about reflexes or the like, but intimate game knowledge and knowing the ins and outs of the whole game. Just because you're good at Starcraft doesn't mean you'll be good at Age of Empires or the like. Skills like base management, microing and macroing etc will get carried over a bit - but it requires a lot more work to get back to the 'competitive' level. But again; those good at StarCraft are generally good at StarCraft 2.
Here's where the lines blend; the competitive level. When it comes down to it BOTH genres require game knowledge and aim/reflexes. Where's the weapons?, how do I counter this?, where's the best place and time to engage?, how long until he gets this and that? It is all the same.
Though I find the same logics apply or do not to other genres. I find myself to be above mediocre at some platform games like Sonic 2 and Super Mario Bros 3 but when I revisit a videogame like Sonic 1 or Super Mario World I am hopeless as I am used to the other game's mechanics.
I eat apples a lot more than pears. There seems to be a lot more of them in shops. Then again, I'm fussy about what apples I eat and I wouldn't say no to any good pear.
Moral of the story: Don't knock something until you try it, I bet you remember being confused at your first try at any genre and I bet you were terrible until you learnt them.