Rhosyn and Roisin compared together makes Rhosyn look like a kreeatyve spelling of Roisin, so I prefer Roisin. Rhosyn likes like a variant anyways with the h and the y.I can't really think of a nickname for Roisin besides "Ro" or "Row".
I personally prefer Roisin, just b/c it looks cleaner. I have nothing against y's, and the Rhosyn spelling would remind me it's not prn "ROYzin" (like a misprn of raisin, lol). Still, I like the look of Roisin.I'd probably go with Sheena as a nn.
I like Rhosyn best. Probably because I have a personal vendetta against the letter i. I'm always trying to replace it with an e or a y. My nickname is Missi and I think that plays into it.
I prefer Roisin because it's Irish, I'd never seen Rhosyn before and it looked contrived to me, but it's a legitimate Welsh name. I still like Roisin and prefer it as a MN in place of the common Rose.