The two options you mentioned, Extend is easier and performace hit is extrememly small, it doesn't require a rebalance as a new container would, but it also doesn't spread your data out more which would allow prefetching to work better. From a managability standpoint, its easier to extend, We have containers that are as large as 20 gig. You would need to decide. Also another thing to look at is, are the containers on there own disk, are you using using a "RAID 5".
If you have fast disk, the size of the containers won't matter as much, you just don't want multiple containers on the same disks.