Swarmlings make getting a very strong army, very easy. The rapid gestation to deployment rate, means that in most missions, you can field a very quick response to any threat. So if you're ever put on the defensive, they're gonna be outright the best unit available.
Technically speaking, I've played the entire game primarily focusing on zerglings. Raptors are certainly fun, and more offense minded, but swarmlings tend to play more towards the Zerg's overall strengths with low-unit costs and high volume. Even against entrenched enemies or outright counters, swarmlings typically deliver enough volume of force that even if quite a few die, there are still enough remaining to kill the enemy units. This is a full 1.5 units to cost ratio after all.
Furthermore, it should be mentioned that banelings also feature a "cliff-scaling" upgrade. Therefore, if you plan to take Raptors solely for their ability to take out cliff defenses, you're better off going with swarmlings and teching into that version of banelings. They're more effective at dealing heavy bursts of damage, which is great for dislodging high-ground tanks, bunkers, what have you where raptors would suffer due to the low damage-per-hit.
The breakdown comes up to whether you prefer to engage quickly, or engage safely. Raptors teched into Metabolic Boost are perhaps the single fastest engaging units in the game. They're quick on land, charge into close range, and deal slightly more damage per hit.
However with Swarmlings, you can tech into things like the adrenal boost and capitalize on the fact that even at worst, some of your troops will still always manage to engage, and with Zerglings, you always tend to have a 2:1 ratio of targets to units.
Personally, I like Swarmlings. They just feel more "Zergy" to me. If I wanted tough units that I could micromanage to win fights a little above their weight class, I'd be playing Wings of Liberty. Raptors are fantastic choices, but I prefer the baneling upgrades to fulfill the role they take. They're slightly more expensive, but in situations where you really need the quick engagement, the few banelings you lose are well worth the explosive damage you deliver.