EXT4 is your only options really if you choose to run custom ROMs. There are a few that support RFS (stock filesystem for Epic 4g) but the goods is running EXT4. EXT4 itself is stable and the ROMs are too, the part that some people have troubles with is the journaling and no journaling on ROMs. It has to do with the amount of times data is written to your device while running.
An easy way to think of it is, running an EXT4 ROM with no journaling is like going fishing without a net. It can be done, its less tackle to carry and more efficient but eventually you will loose that big fish your trying to land. In phone terms your device will suffer a lockup or unforeseen problem and cause you to pull the battery to recover which at that point most of the time ends up with data corruptio.. Then the next time you boot up your met with FC's that sometimes can be overcome but some can't and you end up having to wipe and reflash the ROM worst case.
Now, journaling enabled is like a safety net allowing more data writes to your disk resulting in less chance of data corruption in the event of a crash or lockup. Performance they say is a tad slower but I have never really noticed a difference between the two.
What causes lockups normally is from a kernel not agreeing with what your asking it to do or even that it just doesn't gel well with your device as not all devices are created and act equal. Kernels is a trial and error sometimes to find the one that works good with your ROM choice and device.
Hopefully what I have said helps clear things up. Bottom line you want to run custom you need to go EXT4 and then choose if you want to run journaling enabled or not. Otherwise life is the same as your EVO days, backups and restores and flashing themes, etc and having fun.
Anything else just ask.
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