Kitkat, that's a really tough one. This is your first cycle right? If it doesn't work out, hopefully the next one will be better - often the first is a bust as they try to get things right.
I don't really know, as I haven't been in your situation, but just off the top of my head I'd be inclined to transfer both if they make it to day 3. The whole freeze and thaw things seems a bit iffy (and expensive) just for one embryo.
At my clinic, for fresh cycles, women aged 40+ (I know you're not there yet!) seem to have much better success with day-3 embryos anyway, so I think that's something else to consider as you get to the other end of the 30s.
Hope your little embryos do the right thing