My personal experience. It's all I have.
B3, B3n you plug them and they sound great, solid, bypassed you find back the tone of your instrument and it mixes well with other pedals. The B3 proved its constant worthiness after having being used for hundreds of hours to practice, experiment, record, emergency DI other instruments, whatever is needed. Very solid all metal construction to top the cake.
These units have so far unbeatable quality for the price or even a much higher price.
GT1B, good enough for bedroom practice or spaceship tones but I wouldn't dare bringing it to a gig. It feels weak on both individual effects and readymade patches.
As its name and design clearly imply, the GT1B is based on the GT1, not GT100. At less than half the price, you'd guess they had to cut a few corners. I have no experience with the GT100 though so I can't really compare them.