Same sound except for the GT-8 having speaker/mic sims for DI.
GT-8 has more and far more tweakable effects and can be plugged straight into the desk or used with headphones as a practice/writing tool. It can do everything the ME-70 can do and more.
ME-70 is made to plug into a guitar amp (it has no speaker sims) but is far easier to use as you tweak front panel knobs for every parameter (exactly as you would with stomps), but unlike stomps it allows you to save the parameters in patches.
Tokai SA wrote
The effects on the GT-8 are more than likely as good as the effects on the HD500...maybe even better in some cases.
They each have their pros and cons. Usually (but not always) the HD500 effects sound a little better and are easier to program (sometimes too easy, IMO). The GT-8 has a few effects that sound better than the HD and, once again, are far more tweakable. The amp sims in the HD blow away the Boss ones completely for authenticity.