This is a preference question and a lot of it is what you would be most comfortable working on. There are not a lot of downsides to glass, but some people don't like it because it is not original. I say if you are modding the truck it does not matter. Previously Glass tubs lacked the detail that the metal had so a discerning eye could spot that it was a glass tub, but really only someone that has been around Bronco's and knows the details of the lines. With the Bronco Design tubs that is no longer true, they have worked painstakingly to recreate all of the essential lines and it is a work of art. From what I have seen of it, it is more faithful than the metal repops. Anyways, there are a lot of advantages to a glass tub, first and foremost being rust or lack their of, second would be that it flexes pretty well, in light impacts it will deflect. One of the other draw-backs of glass is grounding, but it's not that big of a deal, you just have to make sure you run wires to the frame and block and then have those wires go to light buckets and something in the cab that acts as a common ground.
Personally, I have an old glass tub, but if I where doing it from scratch again, I would go to Bronco Design and have them build me a tub out of Carbon Fiber and West Epoxy, it would be light, strong and virtually shatterproof. It would add to the cost, but it would be the last tub you ever bought. If I was going to paint it, I would go with Kevlar.
So my point is, either is a good choice, I think one is a little more maintenance free than the other, and most of the real gripes about Glass have been negated by the Bronco Design tub, there are those that still just hate Glass because they think it somehow makes it less a Bronco, but I see glass as no different than repop metal. Unless you are doing a stock concourse resto, their is no reason to not consider glass. It's just a matter of what your preference is. For those that have not worked with glass, body modification can take some getting used to, but once you have mastered glass it is just as easy as welding, in my opinion when it comes to shaping, it's easier but the learning curve is probably higher.
Also if you do decide to go metal, their is the option of picking up the bronco in a box, which is all the repop metal for the Bronco body. They call it the Bronco in a box because all the sheet metal comes in a crate and you weld it together. It's basically a brand new metal body.