I can't say, cause I haven't compared the two, or even heard them... but I would be willing to bet that David Royer knows his shit and that the mod he has going for it is probably fantastic. I would also guess that it kills the v77. Maybe not. I guess we'll see. We need to get some comparisons on the two going, I guess. I've never heard a single bad thing about a Royer mic. Can't say the same about Marshall stuff.
You've lost the biggest point of all though! keep in mind that its not entirely about just HAVING a tube mic, its about getting our hands a little dirty, maybe learning something in the process. That is the most important part of all, don't forget about that!
Keep in mind, also, the Jensen transformer and stuff too. I doubt the Marshall uses anything as nice. The mod is bound to sound better.
$179 with a free mxl603 is a nice deal, though.
Editing in... it looks like the Tube in the Marshall v77 is hardwired in... you can't change it like you normally would change most tubes. No socket. When the tube wears out, you may be in trouble. Also... the diapragm is a 3 micron, which is fine with me, but it might have longevity issues, especially for those people that arent aware that it could be less resistant to handling and SPL damage. According to Stephen Paul, most of the 3 microns out there are across the map response-wise, and it may be extremely hard to find a matched pair if you found the need.