There are a number of options, the rice cooker or crock pot being only two of them. In many regards it will come down to what you have access to and in which country you currently reside.
For all intents and purposes the rice cooker gives you by far the more numerous options with regard to control, and volume. As such, if these are the only choices you're looking at entertaining, the go with the largest rice cooker you can afford. The biggest ones, are usually quite dumb with regard to temp control - which is a good thing.
Each device has it' positives and negatives. For sous vide cooking these are the factors that need to be considered. Heat ramp, how quickly does it heat? Thermal stability, how well does it maintain temperature? Heating accuracy and over shooting, does it overheat by a little or a lot? Thermal shock, when you put something cold into it, does the temp remain close to original or does it need (lots) more heating afterwards and who much?
From all the work done before now, the general indication is that the bigger the 'water bath' the more temperature stable it will be. Next, a PID controller with accurate temp probe is very important especially if cooking delicate products like seafood. Circulation of the water is a definite plus as this helps to maintain a more even temperature through out the bath, whereas non-circulating bath, whilst they do have convection effects, can tend to develop heat zones, as such in this case, smaller is better.
Crockpots are usually, not always, heater by a smallish button heater through direct contact with a ceramic pot. The ceramic pot builds up a thermal mass but because of the, usually, cheap heater and thermostat, can have wildly fluctuating temperature ramps - overshooting the target temp. Rice cookers, because they are usually thin mettal pots tends to have larger heating pads to minimize the potential for scorching, this is benefit for sosu bride cooking as it spreads the heating area over a wider portion of the base of the pot. You can also get electric double boilers that heat water with a built inemmersin heater and then transfer heat to double lidded ceramic pots. Such devices are a great item for hacking as a sous vide cooker as they are already built for handling and heating water.
Other options you might consider is using a PID controller, and moble, water proof temp probe, with a switch box and plugin in what ever it is you want to temp control. This open the options up to using anything that can boil water, electrically, e.g. A wide mouth Birko, or electric jug, or something larger like a commercial Bain Marie - bear in mind, that water ovens, like the Sous Vide Supreme, are accurate, digitally controlled non-circulating (Bain maries) water baths. Their size, and the perforated plate in the bottom, helps to enhance production of convection currents within the device. If you want to do something similar in a rice cooker, get a sheet metal worker to build you a false bottom or tribes out of s/s sheet with 2-4mm dia holes in the sheet.
If you have a particularly, large rice cooker and want to explore the options for circulating the water, there are a number of options. It's been mentioned that you could use and immersible fish tank pump, you could also use a self-priming pump, use a non-contact peristaltic pump as longer as you prime it before use, or any other of a number of pump options. Another alternative it to take a coffee milk frogged - yes, a battery operated coil on a stick and try that, get something larger like a hobby craft paint mixer and attach it to a DC motor, source a long shafted impellor, or recycle the plunger from a single serve French Press. There are many, creative options to try.
Of course all of this is moot, if you don't have an adequate method of excluding air from the bag you're sealing your food in. At it's most simle, is cling wrap and a ziploc bag in a sink of water. A double zip ziploc bag is better, and sealing a smaller ziploc bag wrapped by a larger bag that is immersed in water is an excellent method to consider. Of course, if you can get a vacuum packer, all the better.
Ok, so I've rambled on a bit, so in summary, a rice cooker is better than a crockpot both from a size perspective and from a heating perspective. Get yourself a false bottom for the cooker as this will help with thermal circulation. Keep the size of you food in the cooker in proportion, smaller is better than trying to cook in bulk due to the time required to maintain temperature and regain a stable temperature range.
Yours in Sous Vide
The Baitlayer - A Circus Cook's Diary