It has been claimed that the uBoot does not know how to boot from an ext3 partition, but I have never had a problem here locally. I use ext2 to solid state devices in order to cut down on the number of write cycles, but I use ext3 for conventional disks for reliability in the event of crashes or power failures. If you think about it, it's hard to make a case that searching an ext3 partition is any different than searching an ext2: The former is just the latter with a dedicated logging area. Indeed, you can mount an ext3 disk partition explicitly as ext2 and the OS remains quite happy. (And, if you specify "usb_rootfstype=ext2" in your uBoot environment, that is exactly what
will happen, even if your root partition is an ext3.)
The one potential problem area is if you have upgraded your kernel and then boot without rolling your logs. But kernel upgrades happen rarely and the log issue can be dealt with manually. My advice: If you are using spinning media, I would advocate using ext3 to ext2 in almost every circumstance. Try it, you'll like it. (Caveat: I have no experience with ext4.)