Generally, the "A" series highways are the old routes. For example, 1A used to be the route from Calgary to Banff until the current TransCanada highway was built (Hwy 1). There are portions of Highway 2 that have been replaced or upgraded. The original sections are 2A and the new sections are 2.
The "X" series are usually connectors or small spurs off the main highways. For example, Highway 22 generally runs north-south. Highway 22x runs east-west and connects Highway 2 to 22.
Here's a link that explains it better:
Alberta Highways LinkGiven that the "A" series are usually old routes, you can expect them to either not be twinned, go through towns or have reduced speed limits etc.
The "X" series may be as good or better than the regular numbered roads. 22X is twinned over most of the route but 22 is only two lanes.
