One of the most important factors that you should consider before you purchase a glass water pipe is the thickness and quality and the glass. The thicker and better quality of glass piece you have, the better your pipe is going to be, and the longer it will 
last.
Glass Quality
The quality of your thick glass pipe is important because it will let you know the strength of the glass. You do not want to purchase a low end glass piece that is not of good quality because the glass may be weak and have flaws, which means it could break easily.
The higher quality of glass you have, the stronger and more pristine the glass is going to be. The quality of the thick glass pipe you have is often determined by the material the glass is made out of. It's critical to note, of course, that quality is also determined by the skill level of the glass blower making the glass piece.
You are going to find that most brand name glass pieces are made of high quality borosilicate glass (which is the same material that is used to make glass for lab equipment).
Glass Thickness
The thickness of your glass piece is definitely going to affect the weight and durability of the glass piece. The thickness of a thick glass pipe is usually counted in millimeters. You will find glass thickness typically ranges from 3 mm to 12 mm. Usually, the thickest of glass pieces that you will find is 9 mm, although in many cases this is overkill.
Usually, 5-7mm will be a great level of thickness, with above 10mm being a good call for the base (although again, not totally necessary).
The thicker glass piece you have, the more durable and heavy weight your water pipe is going to be. The heaviness of the glass, of course is more of a personal preference.
If you are looking for the best thick glass pipe that you can find and you have to pick between higher quality versus thicker glass, you should go with the better quality.
You can have an 8 mm thick water pipe, but if the glass quality is poor, it is not going to be as good as a good quality of glass that has a 4 mm thickness. Of course, with a name like Thick Ass Glass, you never have to make the sacrifice. Our water pipe selection features very thick glass, and top notch quality:
https://www.thickassglass.com/collections/water-pipes-1
Thin Glass Can Be Brittle
If you have thin glass on your pipe or oil rig, it is going to heat up faster but the piece is not going to last very long. If you are looking for a good piece, you are not going to want to go with anything under 3.5 mm in thickness.
Glass pieces that are between 4 and 5 mm are going to be sturdy glass pieces but if you want an extremely durable glass piece, you may want to go thicker.