Depends on what you're shooting and why you'd be interested in selling. HVX will have better low light performance, XLRs, standard-def recording, variable frame rates, etc. But it'll also be 2x to 3x as big, uses P2 cards instead of SDHC, and won't be as sharp as the HMC40 when used in adequate/bright light conditions.
If I were going to do this, I would purchase a HPX170 instead. The HPX is an update on the HVX, and for about the same money is a better cam. The only thing you get with a HVX is a tape drive for SD, making it backwards compatible with a DVX100 series. I own both the HPX170 and HMC40 and like them both for different reasons. The HMC40 is very light, more inconspicuous, great IQ, and if you can live with some of the known downsides, can produce some great work on the cheap.
Having said that, the HPX is a superior cam in every aspect except for one, and that is the slightly sharper full raster IQ of the 40. Now IMHO, that doesn't make the 40 a better cam since the HPX has better color and the DVCHDPro codec. The two can match up well together. But for serious work, I would take the HPX every time, hands down. At this point, the HPX is slightly long in the tooth as a model, but it has all the pro features you could want and will keep its position as a economy pro camera for a long time to come. In fact, since the new Panny models coming out in the Fall have CMOS imagers like the 40, the HPX may keep a following indefinitely since many prefer the CCD imager in the HPX. Note that the HPX P2 memory is superior to SD memory, but it is also more expensive. Other things that cost more; bigger filters (72mm) and if you don't have it, a larger tripod and head.
Some things you will like about the HPX compared the the 40:
Better and more menu control
More physical buttons, so less menu digging
Much better VF and flip out!!
Much better low light performance
Better audio
Better codec (IMHO)
Better color (422)
More image control
Real ND switch!!
Grant
Edit: Thanks Barry, I forgot to mention the frame rates
Thanks so much guys. I decided on keeping my 40 as my paid gigs don't justify spending more right now. Decided to stick with the 40 for a bit longer and will assess the situation when some new cams come out.