As you choose which kind of greenhouse you'll buy, you'll also be choosing the type of covering for it. The following list shows the most common kinds of coverings. I have used every one of these, and I'll rank how well they work for growing Nepenthes.
Although it makes for a beautiful structure, especially when combined with a redwood or cedar frame, glass is not the best material for a nepenthes house. The reason for this is multi-fold. First, glass does not diffuse sunlight, which is essential for nearly all species of Nepenthes. You'll have to use diffusers or white wash paint on the outside of the glass, which does not look good. Second, glass does not insulate that well, especially considering the number of pieces of glass that is used to cover the greenhouse. Normally glass covers the greenhouse in 18"x18" pieces. If the glass must cover a 6' long span, then multiple pieces of glass are used, and they overlap each other. These overlap spaces causes a great deal of loss in the "R" factor of glass. You will also have to heavily seal the areas around the glass to keep the insulating factor from being lost even further. Single pane glass does not insulate that well by itself, and with all of the spaces created by the multiple pieces of glass, insulation becomes a major problem. If you live in a climate where it can be very cold, glass can be very difficult to keep heated. Glass is also difficult ot install, and to some degree maintain. Cutting pieces of glass to match a structure can be very difficult. It can be dangerous as well, especially in larger structures. It's also very expensive. Although glass lasts a long time in theory, it will readily break under the right conditions.
There are many different types of polycarbonate, and they come in single pane and double wall varieties. Polycarbonate is a rigid covering, although it is quite flexable, and can easily mold over hoop houses. Double wall polycarbonate provides excellent insulation, as there is a 1/4" to 1" space between the panes of poly. Polycarbonate is often used in endwalls of greenhouses covered with poly film. Polycarbonate is usually good for about 10 years, and it is much easier to install and maintain than glass. Polycarbonate sheets are available in a number of widths and lengths. These larger sheets mean that the overall insulating effectiveness is much better than that of glass. It's less expensive than glass, although much more expensive than poly film. Because of the large sheets its available in, and because it can be easily cut to shape with an electric saw, polycarbonate is easier to install than glass. Polycarbonate is available in panels which diffuse the sunlight.
By far this is the least expensive covering option currently on the market. Although the specifications for the "R" value of film may appear to be lower than that of other coverings, the effective "R" value is usually better. This is due to the poly film being one continuous sheet, without the joints created by polycarbonate and especially glass. In fact, film is such a good "overall" insulator, and is so inexpensive, that many large glass and polycarbate greenhouses uses poly film during the winter as an additional insulation barrier over the standard covering. Film is so inexpensive that it is usually used as a two sheet covering, with an inflation blower which keeps 6"-12" of space between the sheets. An excellent insulating option, covering a 22'x50' greenhouse with a double covering of film and inflation blower only costs about $250. Film is only good for about 4 years, but it is very easy to maintain, and the cost is very inexpensive. With two workers, a large structure can be re-covered in about 20 minutes with double poly film. Poly film is an excellent diffuser of light. Poly film is available in options which are UV protected, as well as types which can prohibit fungus growth. The film filters out a spectrum of light critical for the growth of fungus, which can be beneficial in a Nepenthes house in which there is abundant moisture.
In my opinion, a double layer of poly film is the ideal choice of greenhouse covering for use in a Nepenthes house. For looks, I prefer the endwalls to be made of polycarbonate. I prefer the clear poly film, although the white film may work well for a lowland house. The film does an excellent job of diffusing sunlight, and the double poly film covering with the inflation blower provides an excellent insulating option thats very inexpensive. I only use a 30% shade cloth during the summer months to help cut down on the amount of light, which helps greatly to reduce the heat build up.
Polycarbonate would be my next choice. If looks are important or if a redwood or cedar frame is used, I would definitely opt for the polycarbonate covering.
Glass is the least desirable, in fact, it is actually undesirable for Nepenthes cultivation. Nepenthes need diffused light and temperatures well above freezing, and glass structures make it difficult to accomodate either of these. The fact that it is the most expensive covering as well makes the decision that much easier. A small redwood and glass greenhouse is beautiful, but a film greenhouse 10 times larger loaded with equipment can cost less.