Haven't shot the Baby II but the semi-compact fits my hand nicely. The polymer framed compact 9mm is 9 ozs. lighter than the steel framed compact.
I have wanted to buy the compact version but haven't handled the steel v. polymer framed to see the difference. Unless you plan on firing 10,000+ rds. through your gun I'd think the poly frame would be the best CCW choice.
It's really how it fits your hand and feels when holstered and carried. I haven't read anything contrary about the reliability of the Baby IIs in steel or polymer frames.
"Everybody has a plan until they get hit." M. Tyson