I have 3 gallon glass carboys, a 6 gallon PET carboy, and a 5 gallon plastic BMB with the screw top. I also have some PET plastic buckets.
The 3 gallon glass carboys are pretty awesome. They are thick and I would be genuinely surprised if they broke without a decent bump on a hard surface or pretty serious thermal shock. I bought two of these when I started homebrewing, one went to a buddy to make cider and the other one is used for small batches. To clean the carboy I just rinse it out and then give it a soak with hot PBW.
The 6 gallon PET is what I currently use as my main fermentation vessel. I currently brew 5.5 gallon batches and target 5.25 gallons for the carboy. I always make sure to have a blow off tube handy
I even bought some cool volumetric stickers from brewhardware. I really like how lightweight the carboy is and I'm glad I never have to worry about shattering it. Cleans up great with a quick rinse and then a hot PBW soak.
Can't say many nice things about my 5 gallon BMB, it doesn't seal properly although it is easy to clean. I lent this to a friend who was making a beer back in November ... should probably get it back but I pretty much used it to help haul 10 gallons of RO water for my batches. Now I just use some of my buckets to haul water. I've also used the buckets for fermentation and I've had better luck getting them to seal then the BMB.
I like my 6 gallon PET carboy and 3 gallon glass carboy the best. Why? They seal really well. I can leave my beers in my fermentation chamber for 4-5 weeks if life gets crazy and I don't have to worry about oxidation issues. If you use PBW carboys are also easy to clean out. I just leave them in the bathtub while they are soaking.
After that I would go with plastic buckets. They are cheap, seal decently (especially if you can find a lid that has a grommet - although make sure to keep that clean), and are really easy to clean. BMB comes in dead last, which is a bit funny since it was my most expensive fermentation vessel. I don't have a great opinion about most Northern Brewer products although some of them like my Silver Serpent are actually pretty good.
The Speidel fermentation vessels look fantastic and I wouldn't mind trying one out myself. Congrats on your purchase. Did you decide to get the metal sampling valve?