About to replace some existing 42w compact fluorescent recessed can lights indoors at my office, a few of the ballasts have failed and instead of replacing them ive decided to replace the entire fixtures with an HID equivalent. My local electrical supply dealer showed me a few options as I said I wanted either 50w or 70w metal halide.
 The two I have narrowed down (The fixture containing the Philips CMH lamp is 20% more in price),

70w Parabolic recessed fixture with Philips 70w medium base Ceramic Metal Halide lamp (4000K color, 96 CRI), also contains a halogen auxillary lamp if MH lamp fails to start.
OR
70w Parabolic recessed fixture with Venture 70w coated quartz metal halide lamp (3500K color)

The first option with the Philips lamp actually contains a 70w HPS ballast, so I would only be able to use CMH lamps with it. I know CMH/CDM lamps have way better color rendering and color stability than quartz lamps. But is it worth the extra expense? I have never used either of these bulbs before so I am not familiar with their color qualities, the CHM lamp does have a higher lumen output though

BlueHalide: I would suggest you simply to replace the failed ballasts with new ones, and retain the fixtures. This will contribute to the environment.
A failure of a magnetic ballast, which can be replaced, isn't a reason for a complete fixture replacement.

dor is right: i also suggest to replace the ballast.

however, i'm using quartz and ceramic in various application; CMH are better than HQI
in G12 base (more luminous flux).

42W compact fluorescents are probably TC-T. As far as i know they can work only on electronic ballasts

If there is some light rectification in a HID lamp there will be some 50 or 60 Hz flicker in its output - more pronounced than with fluorescent. So i would NOT recommend to use HID on magnetic ballasts for indoors lighting, in a place where people look at stuff a lot (office), on low ceiling (where lamps are not high enough for the light to be mixed from different lamps). You can end up with lighting that makes people tired and dont know why

So for small HID the option i see is electronic ballats, but then - you won't have advantage over your current TC-T system

So yes indeed repair the TC-T fixtures

The first option with the Philips lamp actually contains a 70w HPS ballast, so I would only be able to use CMH lamps with it.

As far a I know, there is no difference in ballast specification for quartz vs ceramic MH's (speaking about the pulse start only).
So if some ballast is good for ceramic, it is good as well for the quartz.
In Europe the ballast specification is the same for HPS as well, so the same gear may run all three.

And speaking about the lamp type: I would definitely go for the ceramic. It is brighter, should last longer, it is more stable in color and higher color quality.
But the drawback is of course the higher initial purchase cost...

I do agree with the reasoning on just replacing the existing CF ballasts, but I have wanted brighter light in this room for quite a long time now, the ceilings are 14' and the 42w CFLs just dont cut it, three ballasts are dead and the remainder of the working lamps either flicker or are nearing their EOL. The ballasts in the fixtures are Fulham ballasts that are rated for almost a dozen different fluorescent lamp types, they are therefore instant start and it seems I am changing dead CFL lamps every year. The highest wattage lamp type these ballasts are rated for are a "55/65w PL", all 55w CFLs I could find wouldnt fit the fixtures.
I think I will end up purchasing the fixtures containing the Ceramic halide lamps, they powered one up at the supply store and the color looks almost identical to a quartz halogen lamp, very crisp and clean.

Also, a 70w quartz Metal Halide lamp will not last its rated life on a 70w HPS ballast, at least not on CWA gear here in the US (I understand ballasts are different in the UK). It will be overdriven, a friend of mine tried this and the lamps lasted no more than about 1000-1500 hours, one actually exploded after three months

In Europe, 70 watt HPS and 70 watt pulse start MH lamps run off the same ballast, but the same isn't so in North America.  The operating voltage for North American 70 watt HPS is about half what's needed for a 70 watt MH, and the MH lamp will just cycle.

Another benefit with the ceramic MH is that they are less likely to fail as catastrophically as they age than quartz MH.  However, if the fixtures have exposed lamps, I'd use open-rated lamps no matter what type you go with.

@icefoglights, I believe it is the opposite, a MH lamp will be overdriven on an HPS ballast of the equivalent wattage. Whenever I use MH lamps on HPS ballasts they always burn brighter and become a warmer color than compared to operating on the MH ballast its supposed to. In fact I have been running a 250w daylight (6500K) halide on a 250w HPS ballast and the lamp appears brighter and color temp dropped (about to 5000K or so). Lamp has been operating like this for almost a year now no cycling ever. I just expect it not to last that long. The power consumption I get from the 250w HPS ballast when lamp is at full power is 332w, the same lamp on a 250w pulse start halide ballast pulls only 278w

Indeed, the compatibility of MH with HPS is valid only in Europe (maybe except S56, where it happen to more-less match the spec of M102)

What would MH lamp do on an US HPS ballast depend on the exact ballast type.

70W would start faster, but it would most likely extinguish, or run underpowered.
The 150W MH on S55 would do the same as the 70W, on S56 it would run about OK (maybe the temperature stability could be worse)
Higher wattages would be thermally unstable, as the power transfer would increase with the actual arc voltage, so arctube temperature. So it would either not reach the rated wattage, or overdrive the MH. Neither of it is healthy for the lamp, and with some lamp wear (so the arctube retaining more heat) it would always end up overdriven (so by the way more likely to explode)...

If you want brighter light then i would go for linear fluorescents ("squares" for grid ceiling are best)

If you want HID, areas where people dont stare at stuff (corridors, stairs) use magnetic ballasts for reliability, offices etc i think i would go with electronic ballasts to prevent flicker - better replace ballasts once in a while than have people having headaches from 50-60 Hz flickering lights