It's really about weight, mobility, and "features". Vests are heavier, but offer more protection and front pockets. Packs are lighter and offer more mobility, but you have to still wear a transceiver harness and you have no chest protection. Personally, the differences in weight and mobility are small enough that I chose a vest (I have both a BCA MtnPro and a Snowpulse Charger) to get the benefits of having pockets and chest protection. Some people don't like that they can't unzip their coats, but that's never really been an issue for me. Just unzip the vest a bit with your jacket. With the Snowpulse Charger, you can't do that, but it's slim enough I've never really had ventilation issues.
I prefer the vest as well. More comfortable weight distribution and built in impact protection so don't need a techvest especially with the BCA, it has a lot of protection built in. Only time I notice it being too warm is in late spring when most guys have quit riding anyway.