Is a Convoy C8 worth more than than double the cost of an Ultrafire C8 with the the same emitter ?
Yes.
502B has a nice form factor but today you can go wrong with it. Most of them are just crap with modules with hollow pill. Basically no heatsinking at all and heavily driven emitter.
Ok, just pulled the trigger and ordered the Convoy C8 with the XML2-U2-1A emitter from BG !
Can’t wait till it arrives to see what all the hype is all about ! 
You won’t be disappointed. The convoy c8 is a nice light.
My favorite so far. Got 3.5 A. Seems to be heatsinked. Came in Styrofoam box even! Be careful getting C8’s though. Got one with a hollow pill and an XP-E, I think, driven at 1.75A+. Hasn’t blown yet, though.
Also like the 502 B. Got a nice one of those. Switch is broken now. 501 not as nice, but should do the same thing.
Like my sk98. It came with a hollow pill but I stuffed several layers of tinfoil in there & it hasn’t blown yet. Nice light.