20 oz is a good blanket weight, the downside with 24 oz is that they are bulky when folded and storage becomes an issue. Not a problem if you leave it out and don't fold it but summer does come around once a year so something to consider.
I'd buy a minimum of 2 if they are at least 20 oz, taped edges and dark(er) in colour (green/blue/brown/dark grey)...
blake