I can only speak for the TD-3, TB-3 andTB-03 here. I do own an original 303, which means I’ve done a fair few A/B tests - though only against one original, which I think is an issue (because it’s quite clear that 303s can and do differ - mine seems to have a fairly low resonance cap, so it doesn’t squeal quite as much as some other units I’ve heard).
The TB-03 is a nice all-in-one package. I love the oddball reverb, which can give some convincing Plastikman results, especially if you tweak it live. Being able to dial in decent enough Roland FX without extra gear is useful to me. It performs well with what I’d describe as low-key or subtle acid lines, but the accent is very bad (way too heavy-handed) and the resonance doesn’t fare well at the extreme - it becomes ‘brittle’ and unpleasant. For more extreme approaches, I think the TB-3 was better - or at least more able to disguise its failings - but being forced to use the X/Y pad for what should be knob functions was an annoyance too far. I found the TB-3 more interesting for some of its non-303 applications.
The TD-3 does better across the spectrum in terms of sound, though I did have to go in and tweak the filter range pot (which was painless enough). At the factory setting it was going much higher than was accurate, so that anything over the 75% mark sounded wrong (to me) or made no appreciable difference, depending on the other settings. Once that was sorted, the sound was definitely better than the TB-03. I can understand the temptation to put that down to the TB-03 being digital, but I don’t have any evidence for this. The accent is still problematic, but not as much as it is on the TB-03. Accent seems to be the big stumbling block in 303 emulation, and is one area where you definitely appreciate the original.
The distortion on the TD-3 I used once and then left alone forever - a basic Boss Overdrive or Distortion sounds much, much better, even though I’m generally appreciative of onboard effects.
However, I don’t really enjoy the physical aspect of using the TD-3 - it feels, well, appropriate to its cost. I think it’s fantastic that a decent 303 sound can be had for £100, but you do still get what you pay for, and the TD feels hollow and breakable - not in any sense robust. It hasn’t broken, but I’m not expecting it to last (though it might well be easily fixed). The TB-03, in contast, feels great in use despite the small form factor.
So ideally I’d like the TD-3 sound in the TB-03 hardware, but what can you do? It’s very hard to argue with the price of the TD-3 (setting aside any other concerns for the time being). I got a good price on a used TB-03, but if I was paying RRP I’d be carefully comparing it to some of the alternatives. It’s fun, but I think Roland have yet to do the original true justice.