I would also advise checking out Mammoth, which typically has great late season snow. The reason is its high elevation, deep snow pack and predominantly North facing slopes.
Check out these links for some videos of April and May skiing in Mammoth last year.http://www.adventurefilmworks.com/Videos/May7.movwww.mammothsnowman.com
Check out Tony Crocker's snow data at:http://bestsnow.net
For April skiing, seek out areas with high elevation, north-facing slopes, historically deep snowpack depth and a track record of late season snowfall. Tony's site has all of that information.
I am in the same situation that you are in and booked a trip to Mammoth from April 8-15. Our second choice would have been the Cottonwood Canyons in Utah. I've had great skiing in Snowbird in late-April (3.5 ft of powder), but the weather is indeed variable at that time of year. Nevertheless, you should be able to find good snow conditions at either of the above locations in April (could be powder...could be corn...but there will be lots of snow).
In terms of Sunshine AB, I noticed that they already have snow! The snow preservation is very good there (altutude, North facing slopes) and they historically have good snow through April, but they only get about 235" of snow per annum on average compared to 500" in the Cottonwoods and 365" at Mammoth, so you're taking a bit more of a chance on conditions there.
A-Basin has good April snow, but no slope-side accomodations (you need to stay at Keystone and bus in) and it is a fairly small resort.
I don't have an opportunity to ski every weekend like I used to, so I like to stack the odds way in my favor. I'd avoid Sunshine in April unless you can wait until the last minute to book your vacation. If you need to book well in advance, I'd go with Mammoth or Utah.