Blastocyst embryos (usually day 5, but also 6 and 7) are more developed and thus stronger, more resilient, and generally have a better chance of a successful implantation.  This does not mean whatsoever that day 3 (cleavage) embryos cannot and do not make the most beautiful babies.  Generally, if you have plenty of embryos that are developing well we let them continue to day 5 before transferring or freezing. If the number and quality of developing embryos is lower, then we highly recommend doing day 3 transfers as no home (regardless of how great our embryology lab is) is better than your uterus for that developing embryo and future child of yours.