I've been to both sites. When I went to the VLA it was not during an open house. There's more to immediately appreciate at the VLA even if it's to drive up nearby and just look at it with a bit of awe.

The Trinity site was compelling from a historical standpoint but it's really just a patch of desert in a missile range. There was small tent set up with information in addition to the markers. Most of the time for the experience was waiting in a car on a road in the middle of nowhere and speculating about occasional bits of debris in the distance.

Both are worthwhile but I'd go for the VLA.