I cannot answer your question because I have not played either amp, but I am curious how youve narrowed it down to these 2 amps? I personally had a lot of trouble deciding on which amp myself because I liked all of them HRM and non HRM based on sound clips. I ended up getting the FM50 which I will hopefully hear for the first time tonight. I chose that one pretty much by throwing a dart. The only amp I knew I didnt want was the S&M because I heard the switch from clean to OD was not all that great although each channel probably sounds great by themselves.
Anyways I think no matter which one you chose there wont be regret as they are all great amps.
The only thing is once you buy one Nik will release a newer better version right after LOL.

Actually mcinku, your comments on the fendery sound AND being pedal-friendly does help a lot. It's great to have your input! Thanks for the replies

I have not played through a BM, but do have a 50w HRM with the lift negative feedback switch mod.    Flip it one way, you get the signature smooth Dumble-y tone, flip it the other way, you get a bit more top end sparkle/crunch to your clean and overdriven tones.  I can get Fendery or Marshally tones out of it, and it sounds great with el34's or 6l6's.  I run vintage old stock tubes in it as well as the c-lator.  I absolutely love it.  I enjoy playing the one hollow body guitar I own through it, especially on the jazz side of the rock/jazz switch.  When I first got it, I was absolutely floored by the variety of tones that could be coaxed out of it.  Recently it made it's playing out debut amp only, without the c-lator, into a 1/15 Jensen Vibranto filled vintage Gibson cab at a local open mic/jam thang, and at the end of the night the drummer came up to me and with a very direct and sincere look said "you have really great tone." 

A couple of the most well respected forum members here recently responded to your post, and they are right in that you would enjoy either amp.  Main point of mine is that the one I have covers a huge amount of tonal ground very, very well, and I think that it would cover all of your tonal desires as well as any amp out there possibly could.  The c-lator is not necessary, but at the same time it is a wonderful addition, get them both.

Before I recommend anything, I would say: what type of guitars, amps and settings do you normally prefer?
please give a list of your current gear and opinions

e.g. I rarely go for full-on Marshall style OD, but I have amps to do that
I tend to just saturate the amps, or go into a light OD.

With a strat playing neck pickup blues sound, the BM50 is wonderful. I think there is nothing better. Speaker choice is critical though. For me, an Alnico is essential.
Sounds nearly as nice as my Victoria Tweed twin when clean.
Sounds good with all my humbuckers too.
GM50 is a dark amp though, even with Celestion Golds, with volume on 2 or 3 and master vol on 7 (of 10), treble and presence are on nearly max with any strat.
Gets brighter with higher vol & lower master vol though
The high gain sounds are not as Santana/rock as the OTS, and you cannot ride the sustain like on the OTS
For ne, it gets that cleaner Rev Gibbons / Dave Gilmour sound, so well that when I listen to their CDs, the tone sounds weak

I'm starting to think that the normal OTS50 is better suited to humbuckers. Certainly the OD/PAB work best with humbuckers and the bright switch on - PAB is very nasal with single coils and bright switch off

Before ordering, I just described my playing style to Nik, and said who I tend to sound like, and he recommended the BM50. If I said I play it a little more than I play my DC30, or any other amp I own, you'll get an idea of how good it is - but I like Fendery clean tones with a bit of saturation - high gain is something else.
Tell us who you sound like a mixture of, and what guitars you use

Anyway, as everyone says. either would be an excellent amp.
Personally I think the HRM is great, you can tweak the OD, which in my case means taming the extra treble that OD always brings
Also the ones HRM effectively have another way to balance the OD/clean vol, since the master vol is bypassed for OD

You should maybe think of the HRM Mk2, with the EL34s, which are always a bit rockier than 6L6s - probably gets a bit of JTM45 ?
tbh I am not sure I would call any Dumbley amp high-gain though

Wow!! Thanks to all the replies! I should have stated what guitars I do play:
Gibson ES 335
Fender Deluxe Strat w/ Bill Lawrence pup's
Fender Standard Tele

- also an on-going assortment of pedals


I've decided to go with an amp that's being offered which is a 100watt HRM Bluesmaster that has a 1/2 power switch, LNFB switch and a mod to smooth out the OD high end.
Again, thanks to everyone for their input and to the whole forum that supplies such a wealth of information and support. I'll definitely keep up on the posts when I finally get this puppy!