2 sticks are always preferable to 1 for multiple reasons.
1. Redundancy. If for any reason there's an issue and 1 stick fails, you still have a viable stick to get the pc working and diagnose the problem. If you have 1 stick, you have no options at all, pc is bunk until that stick is replaced. That's if you even can guarantee that's its a ram issue and not some other issue.
2. Smaller chips most usually have slightly tighter timings, so a 4Gb stick will work slightly better than a 8Gb stick in certain circumstances. Doubtful you'll see a difference, we are talking nanoseconds, but the cpu will.
3. Most obvious. Dual channel vrs single channel. Dual channel doubles the available bandwidth, which can be upto @ a 20% performance bonus for some programs that use high amounts of bandwidth. On long running programs, 20% time savings is not uncommon and with smaller ram sizes can be a help to alleviate any bandwidth backlogs.
I'd go for the 2x4Gb. Unless you absolutely have plans for one day moving to 16Gb in which case I'd do the 1x8Gb as adding 8Gb will be cheaper than buying a full 16Gb later.