Aguilar TH350 vs GK Legacy 500.

(Edit: I realize you're talking about the Legacy 500 and not the MB500, so my link isn't quite as applicable as I thought it was, though I imagine the Legacy would sound fairly similar to the MB.)

I think you'd be perfectly happy with either one depending on which one has the features you like. Here's a bit of a sound comparison between the GK and Aguilar (with a few others in the middle) if you care to have a listen. Personally, I would lean towards the Aguilar since it has the sweepable mid frequency, but then again, you can also buy an Aguilar Tone Hammer pedal too if that's your thing.

 

The Legacy series sounds more like the old RB series than the MB series. Both have a traditional GK growl inherent in their tone, but more so in the Legacy. I have the 800, the 500 should be more than enough for most gigs.

 

Legacy. More power plus it does the Aggy better than the Aggy does the Legacy.

 

I liked all but the Darkglass. It never sounded warm and clean. The GK and Markbass were my faves. The Magellen stood out as having a different root sound than the others. Either you like it or you don't. I didn't like it's breakup. The Aggie had a more natural breakup.
I'd buy the Markbass over the GK as I preferred the high end. The GK was a bit edgy. And, as the knobs were twiddled, I preferred the Markbass variables in tone.
These are strictly my opinions.

 

+1 for Markbass. I just picked up a minty used Markbass Tube 800 for $500 and couldn't be happier! If I want a solid state sound, it's there. If I want a rich tube sound, I just turn a dial and bam! I'd been going back and forth between an Aguilar and MB for quite a while though. I just happened to spot a MB on the classified I couldn't pass up.