I would suggest doing 12 months because you have flexibility. 18 months is, to me, kind've "meh". Daycare at 1 year is fine, no daycare so being off for 3-4 years til school is fine, but 18 months doesn't get you anything.
If the company is ok with 12 or 18, I'd say to them: We're taking 12 months EI, we'll let you know in ~9 months if we'll return to work at 12 months or 14 or 18 or whatever. Your wife may be climbing the walls after 12 months (especially the last two or three of that!!).