I use 6 gallon Better Bottles for primary and like them a lot - lightweight and safe. I do use a blowoff tube until the fermentation dies down, then replace with an airlock. They also make 5 gallon BB's and, I think, 3 gallon ones.
A few cautions when using Better Bottles:
* Don't use a brush to clean inside - fill with a solution of Oxiclean or PBW and soak - to remove stubborn gunk, after soaking swirl a dishcloth in the cleaning solution - the cloth will do the scrubbing.
* Don't let soak for a long time in too strong a solution of Oxiclean or PBW (less than 1 % by weight, BB recommends 0.5 %.
* Don't use water over 140º F.
* Don't rock the BB on edge to aerate your wort, as this can cause stress cracks - use a tennis ball under the dimple in the center of the bottom, as recommended by BB.
See the BB site for more info:
http://www.better-bottle.com/products_master.html
Also check out some of the broken glass carboy horror stories here:
http://brewing.lustreking.com/articles/brokencarboys.html
and there are some here on HBT, too.
If you decide to go with glass, this may be of interest:
https://www.homebrewtalk.com/f11/glass-carboy-safety-tips-193317/