I own both plus ... Fillmore 50, Fillmore 25, TC-50 and Mark V 35. I've had the Fillmore and Mark for going on a couple years, but just recently bought the TC.I've played both, owned the TC-50. The TC-50 is a mix up of vintage/modern tones while I feel the Fillmore is more of a pure vintage amp to about the late 70s/early 80s level of gain. The Fillmore covers a lot of Fender Bassman & Blackface like clean sounds.
The TC-50 is sort of an evolution of the Mesa Stiletto/Royal Atlantic and other previous 'British' style amps. Its brighter and more tight in the bass than most Mesa amps and is fairly simple to dial in as well. Channel 2 has a lot of classic crunch and breakup, kind of getting into Marshall territory but overall its smoother and more refined to me and not as 'raw'. It sounds more like a modified Marshall at best and as you dial the gain up it becomes more of classic singing Boogie sound. Channel 3 takes it a step further and it becomes more compressed and softer and goes right into a more modern distorted sound with a lot of sustain. It reminds me of the vintage mode on a Dual Rectifier but with less bass.
Both amps have amazing clean tones, the TC-50 I find to be more 'hi-fi' and crisp where I feel the Fillmore is more round and glassy all though both amps cleans change a lot as you tweak the gain and other knobs so this is really just a general description. For me personally I prefer the Fillmore cleans as I like the more scooped Fender clean sound, especially with single coils.. but they are both really good clean channels.
Summary: If you want a mix of British style gain sounds and classic Mesa sound and more modern gain structure.. buy the TC-50. If you want a mix of vintage Fender/Mesa sounds.. buy the Fillmore.
Fillmore is definitely vintage fender-based with some additional gain options. I have a blackface Deluxe Reverb, and I can dial in the Fillmore pretty close. It makes a great pedal platform. The two identical channels with 3-way voicing toggle make it quite versatile. For my purposes, I set it up for slightly dirty clean on channel 1 and grindy mid-gain on channel 2 as a pedal platform. The other way that makes sense to me is to set up both sides as edge of breakup, one side for Les Paul/humbuckers and the other for single-coil as a pedal platform. Regardless, the voice is American.
TC-50 definitely has significant British voicing. The LO channel (middle) has Marshall crang. It's not exactly Marshall, but it's well in that camp. (For reference, I have a 70's JMP 2204 head). The HI channel is still leaning British, but with more mid fullness introduced. This is the Marshall-Mark bastard lovechild. The Clean channel is difficult to define. It's a beautiful clean, but not quite the Fender sonic envelope. The mids are more Boogie. (For reference, blackface and silverface Deluxe Reverbs, silverface Princeton Reverb). I'm classic rock, so I am gravitating to set up the clean for mid-gain sizzle, middle as mid-high gain Marshall and HI as American/British crunch.
The TC is NOT a Mark series amp. I like my Mark V 35, but it doesn't hit the mark for me as a main gigging amp. I have come to the same conclusion that many others have about the Mark V not really being a viable channel switching gigging amp. It would be a good amp to have in a studio, IMO.
Either the Fillmore or the TC absolutely works as a giging amp.