Personally, I'm fond of ASUS and kind of anti-Abit, so I'm not the one to ask as far as a choice between the two. From looking at them they are almost identical. Just remember that if you want to use the Dual channel, you will need (for the most part), matching RAM sticks. I've seen posts that have contradicted this but without personal experience, I can't say. The thing you have to decide is either one 512 w/no dual channel and later adding a second stick to do dual channel, OR going for 2x256 w/dual channel right now. Tough call but if you ever do decide to add more RAM, you could add a third 256 stick and might still have dual channel (POSSIBLY, and Boonesmi or others can correct me here if necessary), but I kind of doubt it.
At any rate, for what you explained as your usage habits, 512 is perfectly fine. Yeah, we all like to have more RAM but sometimes it's more bragging rights than usefull. I do pretty much the same as you but add in a ton of digital photography, using Photoshop Elements 2 and I have no problem with a shortage of memory. I run 512 MB on my A7V333 and 512 on my Dell Laptop.