You cannot deny that we are all generally obsessed with 'youth'. Either it's "legal lolis" or "baby-faced adults" it doesn't matter, the main appeal for both is youth, and it's not just specifically physiological youthfulness. It is hard to really pick between these two cases because if its 'legal lolis', you'll be allured (or alarmed) by the 'number', whereas if its the other way around, you're obviously gonna be more physically assertive towards because it is allowed. Whichever it is, the 'youth' factor is key, it's just manifested in different manners...
And with accordance to social norms, both will trigger a negative response in some way. Like it or, many people will still think there is something 'wrong' in being attracted to either cases. I will not go into that. Basically though, I think both are the same, both are likable.
One can also look at both cases as being their own 'categories' in taste, some fans particularly prey on "Legal-Loli's", at same time, some go for "Baby-faced adults" specifically. Just another thing to consider. Both can be seen as incomparable, different, personal tastes is what I am saying.
as for OT's question:
I really have no preference if it's in the realm of idols, they are all generally pretty, fit, and beautiful. But more importantly, overall taste in girls (physically) will still overpower both cases (i.e. if you like slender boyish types, you'll like slender boyish types, regardless of age or how 'baby' they look). The thing that makes idols great is that it's beyond these physical attributes which really draw you in, in the first place.
But if it's a matter of me being personally/intimately involved, I'd hate to sound like a cliche, but it all depends on how enjoyable being with the girl is. Although being physically 'ready' is a good sign. (Baby-Faced Adults: 1 point)
and to be slightly off-topic: I still don't get whats the deal with Jurina. Being her fan does not make you obliged to be sexually attracted to her.