For films to go out on vimeo, dvd, electronic projection in small theatres:
I’ve been using HV20 to make films and the quality seems superb.
Do I want to switch to M43 or APS-C?
Apart from depth of field (not a problem since I often shoot macro)
what am I losing by the smaller sensor
~ image clarity? overall saturation, which I’d like to call density?
(And less important, would canon G10 camcorder provide better image than hv20?)

Your losing three things:

1/ Ability to get shallow depth of field.
2/ Low light sensitivity.
3/ Moiré. It is very likely that with a smaller sensor size you aren't throwing away a lot of pixels and there is less aliasing and moiré. Yeah, that's a good thing. The other two things are bad.

There is no connection between sensor size and theater release. You need good content. Upgrading your sensor won't change anything if your content stinks.

Thanks for the advice on 'content', Les.
I don't see any arrogance in it at all.
In fact, allowing for the possibility that my content 'stinks',
I'll let your aphorism guide my life.

But beyond that, if my exposure is perfect on a small sensor
you're saying there will be no difference in the quality of the output from a large sensor.

By difference I mean in richness of image. Not interested in depth of field at the moment.
Basically, do I want to buy an XA10 or a T3i?
Based purely on image quality.

Or am I inviting further comment on 'content'?

By the way, thanks for your advice on Carbon Copy on another issue.

Well, between the XA10 and a T3i its a bit of a mixed bag.

T3i would be an interesting option if you do a lot of macro shooting, giving you the ability to use the EF-S 60mm macro and the 100L IS among others (I have a 24mm macro that I love, interesting perspective).

For pure image aesthetics, DSLRs are hard to beat for the price point, but you need to remember that the onboard audio is just junk... basically unusable for any above basic audio needs. Clip length is another thing to consider, but it really depends on your uses.

Scrutinizing detail with DSLRs vs the XA10 I can't really comment on either too well. I would think the XA10 would retain more detail in shots, but I have shot really minimal macro stuff with DSLR and it wasn't high in detail or texture.

Thanks Justin, a lot to think about...
Who makes the 24mm?
I've used a Tokina 35mm which brings object right up to front element...
I've also had good luck with HV20 and simple Raynox adaptor.
That's why I've been wondering if T3i can make it even better.
As long as the image is rich.

Both of those cameras will be an IQ upgrade from the HV20.

Sigma makes a 24mm f/1.8 that I have

Sigma W/A 24mm f/1.8 EX Aspherical DG DF Macro AF Lens 432101

It's MFD is extremely close.

It is very likely that with a smaller sensor size you aren't throwing away a lot of pixels and there is less aliasing and moiré.
True to an extent - but with big caveats. It's the megapixel count that's important here, and yes, lots more pixels aren't normally a good thing. It becomes impossible to read and process them all at video framerates, hence it's neccessary to adopt a simplified system of reading, which nearly always involves ignoring some.

BUT it is possible to get big sensors with not that many photosites, most notably the F3, FS100, and now the C300. Not so many photosites in a big area, and all used, means they tend to be larger individually which means good sensitivity, dynamic range etc.

(And it is possible, in compact cameras primarily for stills, to get the opposite - small sensors with high pixel counts. Hardly surprisingly, don't expect the best video quality from these!)

Do I want to switch to M43 or APS-C?
Apart from depth of field (not a problem since I often shoot macro)
what am I losing by the smaller sensor

As far as video is concerned, the differences are not very perceptible. This is especially true because other 'bottlenecks' exist in consumer and prosumer cameras - such as noise, compression, limited color space, pixel size, etc.

In making your decision, look for features that are more important to your line of work.