I used to own the SX-674 before switching to the ASI-1600MM-C.
First big difference is that the CCD favours traditional long exposures, partly because of the added read noise, partly because of the download time which is 7s in the case of the SX-814, I believe). Some people like this, some people don't. Shorter exposures will accumulate less blurring due to poor guiding / tracking / seeing issues. Shorter exposures themselves have a lot of overhead depending on the software you use though, so you may lose a significant chunk of final SNR if you use very short exposures with high overhead (e.g. on my computer I recently captured 6hrs worth of 35s subs, but incurred about 17s overhead in between each sub!).
I *never* had an issue with frost on my sensor, nor dew on the glass of my SX-674. I've had both on my ASI-1600. It'll be more of an issue in more humid environments (and it's *very* humid in QLD, Australia, where I am). Definite plus for the Argon-purged CCDs here in terms of sensor frost in humid environments.
ASI-1600 has amp glow, which can be calibrated out but adds noise to the image. The CCD shouldn't, and in general will have much "cleaner" images with less FPN. My 674 did show up a lot of cosmic ray strikes though, while the ASI-1600 did not.
You shouldn't need darks with the SX, and the ASI may have some issues with FPN creep reducing the effectiveness of calibration frames. In my experience with the 674 you'll probably still want to dither due to the odd hot pixel.
ASI has variable gain which allows you to trade off full-well capacity for read noise, which can help in getting "sky limited" images in dark skies, particularly for narrowband.
FOV is quite different on the two sensors, whether that's a factor depends on the targets you're imaging.
QE is a fair bit higher on the 814 as well, which will offset the higher read noise. Last I knew the ASI absolute QE wasn't known but was estimated at about 55%, while the 814 peaks at 75%.
SX-814 is definitely a higher quality product, with cleaner images, but not very flexible and favours a system with good tracking / guiding / seeing. The ASI can still produce good quality images, but favours more, shorter exposures (and thus may result in sharper results on lower-end mounts) and produces single subs that aren't as clean out of the box.
Honestly it's a pretty tough call. I shifted from the 674 because I wanted a bigger FOV and higher resolution (in terms of "/px)... if I'd had an 814 I might have kept it instead of going for the ASI... having said that, I am happy with the ASI, and have produced some very nice images with it and would not be inclined to sell it and replace it with an 814 simply due to the cost factor.
Some images I've produced with the ASI in combination with the Esprit 120: