While Mario's answer does a good job explaining the raw differences between the two types of 'ling, I think it bears consideration the possible synergy of both types as well.
For instance, my favorite Kerrigan passive ability is "Zergling Reconstitution". As your zerglings die, you will get them back, at a rate of 10 per 30 seconds.
What this means, in practice, is that you only need to make zerglings once - afterwards, you simply wait for your zerglings to respawn on their own. The combined effect of this is that you will morph very few zerglings in campaign maps.
What this means in practice, is that the Swarmling evolution is of limited use - you can make zerglings fast, and in great numbers... but if your zerglings are continually being reconstituted, there's no real need for either. Reconstituted zerglings are free of resource cost, and do not take up larvae that swarmlings would require. The swarmling's greater numbers are still an advantage (1 more 'ling per moprh), but it's doubtful the 20 or so extra 'lings you'd get over the course of the map would swing the battle much one way or the other. Compared to the bonus damage and leap attack of the Raptor, Reconstitution-heavy strategies have an easy choice.
But even beyond considering reconstitution, there's another idea worth exploring - banelings! Swarmlings let you produce banelings much faster (and at a reduced total mineral cost) than Raptors. If you're a player who's fond of the little, rolling acid-balls, picking swarmlings means more banelings, faster, for less minerals. And while 18 swarmlings on their own might not work as an "Oh @#$*, they're killing my base" button, 18 banelings very well might (and they'll be out more than twice as fast as a Raptor's 12 'ling hail mary). If you're making Zerglings for their explosive relatives, you may find the Swarmlings a better choice, overall.