While the SP101 isn't an N frame it also isn't all that small or lightweight either. 27 oz. empty. IMO the M&P will conceal easier than the SP101 in 3". I carried OWB the SP101 327 for a short time. Compared to carrying an SR40c the Sp101 was harder to conceal due to the barrel of the SP101 would sometimes be poking out below a jacket.
S&W M&P specs
I don't really like carrying on my ankle. Maybe an LCP wouldn't be so much but even an LCR isn't that comfortable in an ankle holster. By the time I got my pant leg up the BG would already have shot me.