I've done thorough testing on random read/write performance of ephemeral vs EBS, and I can tell you that EBS is waaaay better in the case of random IO. I can attest to the accuracy of the random IO performance in one of the references from the post:
Amazon even says this on their page (also referenced by Gabriel).
Joe Stump's article might lead you to believe the ephemeral and EBS are equal for random IO, but Joe only tested on a RAID0 config with two EBS volumes.
In general, even with the best instances under the best circumstances, you won't crack 2K iops/sec on ephemeral RAID0 with four drives.
EBS, on the other hand, with 8 volumes configured in RAID0 will exceed 24K random reads/sec and 12K random writes/sec. The reads are so much higher because EBS is mirrored.
The downside with EBS is you can see worse performance when there are noisy neighbors. I've seen performance drop 50-70% for hours at a time on m1.large because of network card contention, but you can avoid this when on larger instances (m1.xlarge or m2.4xlarge do the trick).
Sequential IO is another matter. I haven't thoroughly compared, but I believe in this case things are more even.
Performance aside, there is something to be said for removing dependencies on a complex system like EBS. It also frees up network bandwidth and provides quite a bit of storage without the $0.10/GB cost of EBS. If iops isn't a problem and you plan on replicating, then ephemeral can be a big win.
http://victortrac.com/EC2_Ephemeral_Disks_vs_EBS_Volumes
Amazon even says this on their page (also referenced by Gabriel).
Joe Stump's article might lead you to believe the ephemeral and EBS are equal for random IO, but Joe only tested on a RAID0 config with two EBS volumes.
In general, even with the best instances under the best circumstances, you won't crack 2K iops/sec on ephemeral RAID0 with four drives.
EBS, on the other hand, with 8 volumes configured in RAID0 will exceed 24K random reads/sec and 12K random writes/sec. The reads are so much higher because EBS is mirrored.
The downside with EBS is you can see worse performance when there are noisy neighbors. I've seen performance drop 50-70% for hours at a time on m1.large because of network card contention, but you can avoid this when on larger instances (m1.xlarge or m2.4xlarge do the trick).
Sequential IO is another matter. I haven't thoroughly compared, but I believe in this case things are more even.
Performance aside, there is something to be said for removing dependencies on a complex system like EBS. It also frees up network bandwidth and provides quite a bit of storage without the $0.10/GB cost of EBS. If iops isn't a problem and you plan on replicating, then ephemeral can be a big win.