Cuddlepie...wishing you luck with this cycle. I am far from an expert with this, as I have only had one stim cycle..I had only three fertilised eggs, and ended up with one blastocyst to transfer and one to freeze. On Day 3 I only had one with 8 cells...so one of my others (I think one was 7, or maybe 5- i just know it was an odd number not even- and one was 4) caught up...I dont know which one ended up being our frostie...but I have a feeling it might have been the slower one, because they actually froze it the next day (after my day 5 transfer) and mentioned how it had caught up.
I understand you thinking that embryos have a better chance inside the body than out, hence transferring the 5 cell and hoping the 8 cell will go on survive to be frozen, giving you two chances.
To be honest, I would probably want to transfer the strongest one though, just to increase my chances of success with the fresh transfer
There is always the chance if one is frozen, it wont survive the thaw.
And like you say, the strongest embryos do make it to day 5

I hope i havent just confused matters further. In the end, I guess your FS will be well placed to give you educated advice and assessment.
I hope this cycle brings that BFP!