I have owned all three. The 6B was awesome, no doubt about it, but if you've got the lust for the bigger and better then hell yeah, it's time to upgrade.
POD X3
Pros
- Great amp sounds, no doubt about it.
- Much better guitar amp sounds than the guitar amps included on the 10B. Not surprising, but it is a lot of fun playing bass through the guitar amps.
- Allows you to have two separate amp paths, so you can play through a guitar and and a bass amp or two different bass amps at the same time.
- Fantastic software for managing/creating patches with your computer.
- Much faster to find a great sound.
Cons
- Not very hardy. The case is made of plastic, and in the initial release of the product, internal components were held onto the case with hot glue, and sometimes fell off and stopped the pedal from working.
- Expression pedal nowhere near as good.
- Buttons feel plasticy and not very stompable.
- Effects are not even in the same ballpark as the 10B.
- Pathetic range of compressors in comparison to both the 6B and 10B.
- Octaver/pitch shifter really sucks in comparison.
- No software offering from Boss, but there is FXFloorboard, which is pretty great (but not as awesome as the X3 software.)
GT-10B
Pros
- Built like a tank. Seriously, this thing rocks as hard as possible for something to rock.
- Unbelievably configurable. If you like tweaking (and you probably do since you own a 6) you will love this even more.
- So many more effects, they are better and much more configurable.
- Designed specifically for bass.
- Way more options in the effect path, and options to split the input signal on frequency, intensity, all sorts of cool stuff like that.
- Assigns, with way more options than the 6 had.
- Built in looper!!! It is much better than the one on the 6.
Cons
- It has two separate effect paths, but you can only have one amp sim. This really pissed me off when I bought it.

- Nowhere near as many amps. However, the bass amps on there are GREAT.
- Takes tweaking to find a great sound.
In conclusion
GT-10B hands down. It destroys the X3 for bass. You will never be satisfied with the limitations of the X3 after you've been used to the configurability of the 6B. The 10B is even MORE configurable! And it has a built-in looper, which you will never stop playing with. Also the looper allows you to create patches really easily, because you play a line and loop it, and then start tweaking options (which I'm pretty sure you couldn't do on the 6). However, and this must be emphasised for a fair comparison, the X3 really really does have amazing amp sounds.
Ask me any questions, I've played with them all extensively.