What are the merits of ext3 over ext4?

Ext3 has been around since 2001 and is a logical extension of Ext2. Ext3 is regarded as being very reliable, and is available on virtually every Linux distribution. It's main feature is the ability to journal writes to the filesystem, allowing much faster crash recovery times, as there is no requirement to fsck the filesystem before mounting it.

Ext4 is a recent revision of Ext3, which adds the ability to store large (multi megabyte, ideally gigabyte) files more efficiently using an extend based layout. The expected benefit from using Ext4 over Ext3 would be a lower accounting overhead (less blocks on disk dedicated to filesystem metadata) and an increased throughput in sequential IO as extent based filesystems tend to group data physically together, reducing internal fragmentation.

I think the main advantage of ext4 is probably extents which is good for storing large files (video). However, ext3 has been around long so my instinct would be to trust it a little more.