You're asking for what we epidemiologists call the "counterfactual". Since one can't make exactly the same shelter twice with different kinds of seams and compare the weight (because the small difference in seam weight will get lost in the large variability in shelter weight), you won't get any quantitative responses to your question. You'll get differing opinions.
My opinion, for what it is worth, is that bonding is both stronger and lighter. When you bond, you add an extremely thin film of adhesive between smooth layers of mylar. When you seam seal after sewing, you add a comparatively large amount of adhesive to the irregular, perforated surface of a sewn seam. So sewing adds the weight of thread plus a relatively large amount of glue, while bonding adds only the weight of a small amount of glue.
As far as strength is concerned, I'd recommend reading the many posts in these forums about that. I think there is a growing consensus that bonding (done well) is much stronger than sewing, but others might disagree.