It stinks there isn't a crystal ball for this stuff. My advice would be to proceed in a way that'll leave you as few regrets as possible regardless of the outcome. Lay it all out on paper:
If we do this and that happens, I will feel _____
If we don't do that and this other scenario happens, I will feel ______
That helped us narrow down what we both felt most comfortable with.
If it helps at all we tried to fit everything we planned to do in by the end of '13 and couldn't. An out of the blue bonus check in January allowed us to take the plunge with our IVF in spring of '14 and because of the expenses that bonus check didn't cover - the subsequent OB care, delivery expense and first month of newborn visits were all "free." Depending on where you're from you'll probably end up paying out one end of the process or the other. January/February would set you up for a one year wham bam, knock everything out in one tax year as opposed to meeting deductible's twice over two years to cover both.
(Money is usually a factor for couples on some level, but if it's something you could scrape together and need to do so later on down the road don't hesitate. It is sooooooo expensive but even now hubs says I can't believe we almost didn't because it was unfathomably hard to write that check for a mere chance.......he says he would have regretted not doing so the rest of his life. Even if we hadn't fallen pregnant I know we both would have always wondered).