FXAIX vs VOO in Fidelity taxable account for portability

Folks - it's possible we create a special needs trust (either at death or during lifetime) but I need to research that well first. Child's only 7 yrs old so a while to go but you never know what life brings on so we may set one up with minimal funding for now. My main - is VOO better suited than FXAIX? So far, we are only invested in FXAIX in Fidelity taxable accounts but thinking of switching to VOO given it's more portable to vehicles like SNT's. We may want to add future cash into VOO to bring it up to a 70/30 FXAIX/VOO portfolio long run. I know expense ratio & tax efficiency is very similar. Thanks