Day_dreaming, here is my view on this. I used to be a big proponent on D5 transfer because I thought we would let "nature" (or lab) select the strongest embryos. However, my view has changed lately. I no longer think Day 5 transfer is better, although it is still possibly going to yield a slightly higher pregnancy rate per transfer.
First of all, the lab is not our womb. There is no way to say that the strongest embryo in the lab will also be the one that will perform the best in the womb. The "natural selection" is not in a natural environment.
Secondly, it is clear that there are many embryos that could make it in the womb but could not survive from Day 3 to Day 5 in the lab. Why do I think so? If you look at clinical pregnancy rates in the US, you can see the rate of success of Day 3 is maybe 30% and Day 5 40%, in very general terms. So the difference is only 25% proportion-wise. However, the rate of demise between Day 3 and Day 5 in the lab is about 50 - 60%. That means a significant portion of the Day 3 embryos, maybe around 30 - 40%, would have been able to survive if they were in the womb but ended up dying in the lab.
Third, there is also a factor of selection bias that could explain the different success rate between Day 3 and Day 5 transfers. Usually the women doing D5 transfer have more embryos. So if you were to choose 2 out of 10 embryos, you are more likely to get a higher quality embryo than from a smaller pool. So maybe culturing to Day 5 was not the key, but having a lot of embryos to choose from is.
Costwise I think it's a wash. Doing blast means fewer frozen, so there is a higher chance of having to do a fresh cycle again.
That's just my view at the moment. It could change with more research.

The most important is to trust you've made the best selection possible and not second-guess yourself. If you think too much, whichever you choose, you will wonder if the other is better.
Let me know what you decide! I would love to hear your thoughts too!