I'm an idiot who gets Suica card then returns it at the end of each trip. I'm just afraid that it won't work when I return a few months later. I do the same with Octopus. We've discussed this elsewhere in this forum... it's irrational, I should just hang onto it. I just lack faith in these IC cards' durability, because hotel room cards crap out all the time.
Anyways, I always get Suica. Never Pasmo. I just think more people get Suica, even though there may ultimately not be too much difference. One definite merit of Suica is that you can accrue points when you pay with Suica. Places like Ito Yokado, Kinokuniya, and a lot of the businesses in the JR paid area of the station (eg, Ecute). Granted you don't really accrue much value through this program, and I'm not sure if there's an English website that walks you through the registration and redemption process. I don't think Pasmo has such program, although I could be wrong. Another benefit with Suica is that there're attractions that offer discounts to Suica holder. I just learned this the other day when I went to the Rainbow Pool at Tachikawa where the admission was 10-15% less just for showing Suica.