While some of this is recap of what other folks have already said, I've owned several G-Majors and several GT8's over the years, so I feel morally obligated to weigh in...
The G-major has a solid base of effects (modulation, pitch shift, delay, reverb). In general, the quality of effects is good (the phaser is probably the most disappointing) and relatively transparent. I've never found it difficult to dial in decent sounds. To change patches on the fly during performance, you would need some sort of MIDI foot controller (perhaps you can use a standard 1/4" footswitch to do patch up/down, but I've never tried it).
The GT8 has a far wider range of effects in addition to all the amp models, although I don't find them to be as clean or transparent. Using the 4 cable method, you can put some effects in front of the amp and some in the loop. IMO this is the coolest feature of the GT8; however, if you read the manual, the GT8's loop is designed for instrument level (not line level), which is why (as Pete mentions above) you'll want to pick up the Ebtech line level shifter. Otherwise, the GT8 can suck the tone and feel out of your rig. Also, the GT8 has a great PC-based editor, which is helpful for dialing in the effects since there are so many options and parameters.
Both units have relay switching, which should allow you to change channels on the RM50 using a 1/4" cable. This is immediately useful with the Boss (since it is essentially its own foot controller) but this is where you would benefit from a separate MIDI foot controller to make the G-Major shine.
Net, I like both of them, for different reasons. So I'd say "which to choose" depends on several factors...
1) Do you want a lot of effects to play with (Boss) or just a few high quality effects (TC)?
2) Do you care about being able to have front-of-amp effects like wah, overdrive, distortion, compressor (Boss)?
3) Do you plan on adding additional rack gear in the future? If not, do you want to lug around a rack for just one piece of gear? Perhaps an all-in-one floor unit (Boss) will be easier to manage.
Hope this helps... I actually just picked up another GT8 to use with my RM50B. In the past I've had major tone suck issues with it, so I'm hoping the Ebtech box will make all the difference this time around. Plus, the GT8 has never made a lot of sense with my rack rig (if I'm already going to carry around a rack, why not use a G-Major or Lexicon unit?), but I've since stopped using FX with my M4 rig (sounds too good on its own).
Also, I kind of miss the models in it for recording scratch/demo tracks... I'm not the worlds biggest fan of modeling devices, but the ability to plug the GT8 directly into the console to throw down some quick tracks is great.
--B