Cut out paper in the 34” and 28” and place / tape each anove the vanity; then see which one looks better.

That is the first step for sure but IMO the stlye of the bathroom and the mirror need to work together some maybe some pics

It also depends on what your sconces look like (how big are they?). Taping up a paper model of the different sizes is the best way to figure it out as @kculbers said. Also model the sconces when you're looking at the mirror width. Even if you're not 100% sure which sconces you're buying you should still have an idea of what you want and how wide and tall they might be.

Wouldn't it be better if it's at least 42-44 inches? I would like to see some picture of the bathroom and the vanity too so we could give you the best recommendations